So, I don't have a plywood-backed bench seat... How-some-ever, I do have a plywood splitter on the nose.
I used 1/4 inch plywood, the cheapest one with smooth (no football shaped holes, etc) surfaces. Since I was using West epoxy resin for the fiberglass bodywork, I had lots of that stuff hanging around. I "painted" both sides and the edges of the plywood with West epoxy, let it cure for a day or six and then painted it.
The car is garaged, so I can't say the wood/epoxy has withstood much weather. It has been wet. It has been out in the sun all day. And it's hit a few bumps along the way. Some repainting has been needed on the bottom side. Otherwise, it held up just fine. As a seatback, covered, somewhat protected, I think it'd work.
Alternately, you might try some good mid-grade plywood and some of the "Porch Sealer" stuff they sell at big-box home supply places.
JDK
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JD, father of
Quinn, Son of a...
Build LogQuinn the Slotus:Ford 302 Powered, Mallock-Inspired, Tube Frame, Hillclimb Special "Gonzo and friends: Last night must have been quite a night. Camelot moments, mechanical marvels, Rustoleum launches, flying squirrels, fru-fru tea cuppers, V8 envy, Ensure catch cans -- and it wasn't even a full moon." -- SeattleTom