Making the longerons, forming the sides.
Cutting the curve on FJA was just about as easy. Mark the 2" spacing, pencil in perpendicular lines with your square. Mark the cut point on each line by setting your dial caliper to the measurement in the drawing. Most calipers will do inside, outside, depth, and STEP measurements. Use the step measurement process to make your marks. Now this is trick... Take a long length of 1/4" plywood and use it as a bow to mark the curve. I used a piece of 6mm Baltic Birch, 60" long and 1.5" wide. Radius the corners on the ends so the sharp edge doesn't cut the tape in the next step. Set the board on the FJA stock, bend to a curve intersecting as many of the points as you can, run a 70" piece of fiber strapping tape to hold the curve. FWIW, my chord height was about 2". Other stuff will work too, make sure any grain is straight and uniform. A piece of metal strapping, ...
Now over to the band saw to cut just shy of the line. Setup the sleeve sander (3") on the drill press, finish sand to the line. Knock off the minor bumps and imperfections with a small flat sanding block (it'll follow the radius close enough). Trace this curve on the other three boards, bandsaw to .125" shy of the line. Finish with the .5" bottom bearing straight router bit (laminate trimmer bit) in the router. Tack the master on one board, rout, repeat for the other two.
I ripped a handful of 1" strips from 6mm Baltic Birch (the firewall material) for the longerons. Eleven strips 60" long as I recall. I'm using birch plywood for several reasons. It's dimensionally stable, I have a ton of it, the plans Sitka Spruce gets slotted several places so I have to assume the longerons are not exactly structural, the ACS catalog mentions "cap strip" may be cut from 'generic' spruce stock as it's non structural, I saved on shipping, and I've read where some builders rip up a 2x4 from the Big-Box-Store for longeron material. Disadvantages are it's heavier, it doesn't come in 105" lengths, the finished three strip width is .70 inches. Note the staggered joints (a scarf joint would be ideal), and if you've got good eyes, the slight gap where the longerons meet the junction of FJB and FJC. Also note the shims ripped to 1" from shim-shingles (roof shingles) under the clamps where the curves are extreme to keep the clamps from slipping. I used 12 clamps FWIW.
I ripped the 1/8" Masonite to 20.75", then added 6" to the end to make it 102" OAL. I cut the side foam to the dimensions in the plans, a strip of 6mm Baltic Birch worked well to get a smooth curve between the points. Welded the FJx jigs to the table with Mondo-Bondo (probably should have used more), then glued the foam to the Masonite with 5-minute weld-o-matic in the ever popular dual squeeze bottle. I used drops of 5-minute, maybe the size of a paper match head, placing the drops near the edge where the foam will get sanded away during contour in a later chapter. The foam lays down pretty well on it's own, a few spots of glues helps.
I routed the stick depression, and the depression in the back of the boat for the fuel sight gauge. I'm using the Vance Atkinson gauge, a real sweet unit BTW. While routing the stick depression, Britney Spears called and asked for a date on the speaker phone. I slipped with the router and gouged down to the masonite. A sliver of foam, about 3/16" thick, placed over the boo-boo. Use the Xacto knife to cut through both the patch and side foam. Lift out the mistake, press the patch in. No micro on the edges, I figure It'll get a little that flows down when I glass up the inside. Sand to the surrounding level, even Stevie Wonder can't see the repair. I told Britney I was busy.
You may notice the spacer foam extends slightly 'long' of the side foam. Compound curve and all, some builders have come up short in the foam department when they add the lower longerons. I don't care (mostly) about the excess overhang, I'll be measuring the exact side contour when I place the triangular longeron, as the bottom of the tub locates against these. I can sand any foam overhang after the lower longeron cures. If this comes to bite me in the posterior, I'll delete this paragraph. Shhhh....
I cut 102" of UNI from the roll, folded it in half lengthwise, then in half again. Made sure the fibers were straight and the folds 'neat', brushed a heavy coat of epoxy on the upper longerons, then carefully placed this four ply sandwich on top. Drizzled on more epoxy, brushed, used my fingers, poked the corners with a stick. Cut along the gap between the two sides, as well as trimming with the scissors to a 1.5" lap at the lower edge once totally wetted out and squeegee' d. Worked out pretty slick! Don't forget to radius the inside (only) edges of the longerons beforehand. The top edge of the longeron remains square.
Made a jig to trim the bow end square using the Binford high torque five speed soft start router, and a super magnum diamond honed micro-grain Unobtanium Carbide extra long router bit. The picture should give you an idea how it went together. About a 10 degree bevel on the legs. You can see a drywall square laying on the longerons in the background, used to get the jig square before clamping. Flip the square over and measure again. Often they're not exactly 90 degrees after being used (dropped, stepped on, ...). Split the difference if so.
I'll do something similar at the exhaust end, once the 6 ply BID over the foam is added and has cured.
I took some time to trim the bulkheads to fit the sides too. F-22 fits perfect, and the seat back needed the corners trimmed (this was planned) But like some have found, my IP is a bit short on the bottom, maybe 3/16" at the outside corner. I think the center should be OK, I didn't make it flat, rather followed the slight bulge in the center per my full size "M" drawing. Looks like two loooong tapered wedges of foam are in order. I'll know more in the next chapter when I jig all these parts together and add the bottom.
I sanded (router skid per above wasn't used) the foam and glass at the lower longern to be square with the face of longeron, though question the effort. This longeron has a compound curve (curving in) near the aft end, but the bottom foam assembly in the next chapter is fitted in at 90°.
I do think cutting the spacer foam long at the bottom (per above) was an advantage.
Only defects I noticed were a slight void where I glued the foam spacers in (near the top of the seatback), a larger than preferred flox joint in a few places along the top longeron glue line, and a few ripples in the inside glass work. Nothing a heavy coat of Zolatone won't fix...