I want to do something to the outside of the marine plywood (TPO, EPDM, aluminum, Skittles?)
Whatever you do seal the ply with epoxy...Which I assume you will use (thickened) to bond the two layers.
A 6oz or maybe 4oz layer of fiberglass sheathing bonded with epoxy would be good - very common approach on boat cabins & hulls - not too hard to finish nicely as the weave fills easily with faring bog (easily sanded epoxy/filler mix. If painted with two part LP it's as durable as gel coat.
I've bonded alu sheet to epoxy sealed ply with sika successfully - it would be heavy & how do you deal with the corners? Maybe sika angle alu for the corners but that curved part will be a challenge. Great thing about this approach is NO Sanding...Sika makes a 291 LOP (Long Open Time) which I strongly suggest for all your sika bonding, the regular stuff sets up too fast for what you're doing. To get the alu to lay down nicely on the ply is the challenge and generally you only get one shot at it as the alu sheet often distorts if you try & pull it off to reposition. You can lay it down oversize & router off the excess...The curved part will bond great but the flats will need lots of evenly distributed weight & cleverness to avoid screws - I'd think screws on the edges where you have to hide the joint anyway would be a given - you might plan on roof racks/solar/vents/lights on the roof to give you a place to resort to screws...If you gently pound on the sheet as it goes down that helps the sika to flow & bond.
Why 1/4 luan on the inside? Okume is nicer & 3/16 (5mm) would be fine if you bond it to the foam...
Great build & thanks for sharing. Moe