I used plywood, no rip utility fabric, fleece, memory foam, and a staple gun for this project.
Measure the crate and cut the board, foam, and fabric to fit.
Staple down the no rip fabric tightly to the board. This will hold the foam to the board.
I cut a fleece blanket in half to cover two boards for both crates and stapled that down to finish it off. I didn't bother making the underside pretty because no one will see it (besides you, haha).
She loves it already!
If you don't quite trust your dog in the crate if they aren't potty trained you could use waterproof fabric underneath and make the fleece similar to a pillowcase slip so it could be washed.