So, start with 3 "standard" estes 24mm (D and E) motor mount kits (p/n 303159). I built them with Epoxy, instead of white/yellow glue, just to insure strength, since this "Could" be launched on 3 F39's !
Attach 1-1/2 yard of kevlar shock cord to 2 of the mmts, and 3 yards to the mmt that will be the Center mmt.
Mark both of the side pod tubes as if you were going to attach a normal 3-fin layup (ie 120 deg) starting at the base, draw the fin lines 4 inches up the tube.
-now figure out some way to cut out 3/16" slots over those lines, for the Through-the-wall fin tabs.
Cut out 4 fins with TTW tabs out of 1/8 basswood or birch. Also cut out rectangles measureing 4in x 5/8" (current guess).
mark on the TTW's where the Motor Mount centering rings will be, and cut the TTW's as needed to get a flush fit against the center of the MMT's.
Spot tack just 2 Fins on one MMT and 2 more on another. Spot tack one rectangle where the 3rd fin would be on each mmt. (yes, after cutting out the material where the CR will sit)
spot tack the free ends of both rectangles to the 3rd MMT (the one with out anything glued to it yet).
Test fit the MMT and the body tubes. Don't glue them in, nor glue the Bt-55's together, yet. Make adjustments as needed. The Aft CR's should be inside and flush with the aft of each BT-55.
once everything seems good, remove the mmt from the body tubes and layup that MMT assembly with Fiberglass.
What? fiberglass?.. ok, it really isn't nessasarly, I admitt. But for the Scaled up 38mm version it will be needed, and I need the practice.
Note: it would have been easier to attach the 3 mmt together first, with the rectangles, THEN to attach the fins. As the three mmts could lay flat on a surface to insure perfect alignment. Noted for next time.