By one sheet of Advantec 3/4" plywood at Lowes. It is guaranteed for 50 years and is glued with apoxy in the manufacturing process. Double it up and skin it with aluminum if you want too. I've used it in houses and fly-bridge repair on large boats . Strong as a bull and will give you the strength you need for any motor size that won't sink the boat.
Did some looking on Coosa Board and also found King Starboard. This stuff looks tough. I don't mind stepping up to the plate when it comes to doing it right the first time. In my case it would cover the entirety of the transom which is about 6' wide. I wouldn't feel bad about covering it up with 1/4" aluminum if I don't have to ever worry about rot.
My guess is that many new boats are using Coosa Board in the transom. I considered using it on my DR, but only find available in 1/2 sheet min size. If you bought a large sheet I would like to buy a piece of it from you.
Currently I have 1" plywood at transom (rotted out in places) and a 2x10 @ the rear motor bracket the engine sits on. Been looking at new aluminum boats (not that I can afford one) just to see how they're doing it. There's no wood anywhere. The salesman said there's bracing encapsulated within the aluminum skin. He didn't elaborate on what was under the skin but there was a big, fat ass Merc 4 stroke hanging on it so it has to be strong. I have long shaft Merc 500's, Merc 110 nd a recently acquired 150 horse Merc. Not sure if I'll go with a couple Merc 500's or a single higher horse Merc. I'm a do it once knda guy, even if I have to bend over to make sure it's done right. Wood will eventually need to be replaced, marine grade or not. I figure with proper diet and exercise I'm good for another 50 years. I'm keeping the boat and I don't wanna be replacing rotted wood when I'm 90. I'll continue to research.
There isn't any way the 1/4" aluminum would have near the strength of the 1" plywood. Plus you are losing the dampening effect of the plywood. When the frequency of the motors sync with the harmonics of the hull,,,,your teeth will fall out.
Wisdom may come on little cat's feet but in so doi
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Stop dreaming and get out in the driveway and start working.
Once you start, you will see the folly in what you are proposing.
I am not an engineer either but there is no way in hell you will convince me that a cobbled together 3/4in box section "frame" will be stronger than good old 3/4 plywood. Get real.