Cabinet door material

Or maybe just use quarter inch Baltic Birch plywood and use a forstner bit to cut a few holes for ventilation??

Well off to the big box store again, lol. I should have my own parking spot there.

Shutter doors.

Home Depot carries sheet metal in 36 x 36 punched out decorative patterns. It can be cut to size with a pair of tin snips.

Clicky

You can find something similar in the form of a pet guard for a patio sliding screen and cut it to size

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.

Why do you need expanded metal?