Anything goes. I used 4mm plywood to build a recessed board and it worked just fine.
On the downside, compared to cardboard, you need a saw to cut it, but not very much more (just some glue), but on the upside it is more stable than cardboard and it stays in shape even if it gets wet.
You just cut the board so it fits the camera, cut out a square (or circle if you prefer that) and build a box behind it and glue to the back of the front board.
Make sure the hole in the front is big enough compared to the picture angle (and you have yourself a built in shade as well).

As I said, anything goes and good luck.