I've used the boards they sell for mats for pictures. They have other good boards in the craft stores near the picture frames-can be cut to any size & you get a lot of mileage out of 1 board. Then cover with a sleeve, but you don't have to sew it closed - just make it longer & fold it in. Easier for washing!

I use the stiff craft interfacing, I think one brand is called Timtex. I've heard people recommend buying the stiff place mats at the dollar store but ours doesn't carry them.

I use 4" x 14" x 1/4" boards from Micheals. Usually on display with the balsa wood.

This is my choice also. You forgot how easy they are to cut.

I've used plastic canvas and I've used the plastic folders and made a sleeve. All good suggestions.

I use a heavy mat board. It comes in different weights, and I get it in the art/frame dept. at Hobby Lobby or JoAnns. I have also used plexiglas. I got it at Lowe's, and they cut it to size for me. I bought one pre cut size, and they allowed two free cuts from that. I make a fabric sleeve for both these products, and they hold up very well.

Does anyone have trouble with things going under the sleeve? I did.

I have had wonderful luck with this method. Being able to remove it for washing is super. I can take the foam board out and just wash the sleeve. Had a problem with things sliding under it so use a spray glue when I put it in the bottom.

You know those Bag-E-Bottoms that are super expensive, well I get the local glass shop to cut me plexiglass in the sizes I need for a fraction of the cost.

I use foam boards, I buy at the Dollar Store. You can cut several from one board. Plus, I save used rotary
blades to cut them. I use 2 boards, taped together with duct tape for extra strong bottoms. I do cover
them with a fabric sleeve. I've tried other suggestions and this is the cheapest way I have found.
Hope this helps.