I didn't really have a clear idea. I removed the upper metal door parts to deal with rust and got thinking that I could cut and fit a nice wood panel in there and put the metal cover right back on top and it would look much better.

I didn;t have any particular cars in mind. any suggestions?

I'm a bit of a woodworker myself, but I don't see needing much here beyond a scrollsaw and a drill press. The issue that I'm wrestling with is wood itself. I'm thinking that 1/8" or 3/16" is the way to go, but that thin, and the selection is limited. Not sure where I could find a nice piece of 3/16" or thinner oak or birch vener plywood. There would be ply edges to deal with as well. Of course there are clearance issues around the door that shouldn't be overlooked either.

I'm thinking that aside from finding the wood itself, it wouldn't be too hard. Just 3 panels per side, so 6 total. Maybe something with the dashboard if needed., maybe a panel on the floor of the cargo area.
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You mean Volvo makes cars that are *NOT* Wagons?!?