The more wood you use the heavier it will be, wood on top of the alum framing is okay, but remember the plywood will soak up water like a spnoge and if you seal it, it will not escape normally and hold it longer causing rot, been there before, pressure treated wood has a chemical that will attack aluminum so dont use that, battery acid is bad for aluminum also so create a stainless steel or plastic containment for the batteries, place any wiring you may need below the floor, we added styrofoam sheet between everything to make up for seats being gone, consider load placement b4 you start, the tounge of the trailer will transfer any weight you add right to you tow vehicle, it will also take a super human to pick up the tomgue or move it around if you load too far forward, i saw a rig that badfish has and his battery weight is offset front and rear of his trailer axle, that was real smart on his part, you may want to start trying to make friends at a local sheet metal shop to have aluminum sheet cut and formed or find a scrap yard that will let you root thru the pile, ours is not friendly that way, had to go new, you will need to buy a good size box of 1/4-20 bolts about 1-1/4 long at a commercial distributer not home depot or lowes, they will rip you off, you should beat their price by at least a third, but go in with cash for the hardware and know what you need before you get there, use nylon lock nuts on the inside and apply silicone forst, use this combination to add any angle framing you may need, it will not leak. the commercial hardware place i go to has a lot of folks with saltwater boat buy from them so they are cool and very helpful as far as new advances in hardware, you may want to give them a chance to offer any knowledge they have in this area.
this was the last boat, hope to make the next one better