Or maybe just use quarter inch Baltic Birch plywood and use a forstner bit to cut a few holes for ventilation??
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation) 2013 Toyota Tacoma Off-Road (semi-beefy tires and components) Our trips -- pix and text About our trailer "A journey of a thousand miles begins with a single list."
I assume you require something that permits air circulation?
Big box hardware stores have various punched metal sheets. You could also likely find various forms of louvers and heating registers that could be mounted in a solid panel of plywood or whatever.
If you want an early American sort of look, maybe a sheet of tin punched with nail holes (and possibly also embossed) in some decorative pattern would be good, such as might be found on a pie safe.