I'm re-thinking the basics again. I can't get trustworthy 1/8" plywood locally, sqweaka's work opened my eyes and the KISS principle all have me leaning this way:
A-Frame of rectangular tube or C-channel in the 2.3#/ft range, with short bunks on each side for the Dexter 8, and cabin mounts. 1/2" floor with some transverse spars and perimeter framing, extra beef at the attachment areas to disperse the punch loads.
1/2" walls (maybe 1/4 or 3/8) with door/window framing, gussetts at joints, and FRP cloth/resin over all exposed seams.
1/4" roof over exposed spars (rounded edges), but maybe a 1/8 luan headliner makes sense. Unsure. The front of the roof from floor up a foot or so will be fabbed as a structural beam to take the punch loads there.
Very basic, dry galley; cubby holes, no drawers, sinks, ranges or that level of equipment. Hot plate, plastic wash bins, toaster oven. 110vac for interior lights, fan, galley appliances.
Thats what I'm thinking anyway. Easier, quicker, simpler and suitable material looks available and fairly inexpensive. My estimate is a little under 600# for Meranti or DF, 500# with Okoume.
I hope to get some good plywood news from a nearby lumberyard tomorrow. With luck some exterior, but not marine, Okaume will be available in 1/4 and 1/2 at an attractive price.
Many thanks to all of you for the ideas you've shared on and off list!