My friend Angie's husband made a baby crib and needed a mattress for it. I use 1 inch form 35.5 x 21. Then covered it with rubber sheeting and added a zipper to take off and wash. The foam and sheeting is so expensive. I couldn't believe it. Glad I had two 50% off coupons, but still too high.
Then I made her 2 sheets, because I thought it was an odd size. Turns out I had 2 little vintage clown sheets that fit perfect. I had sheeting and fabric left over so I made her 2 lap covers. Turned out pretty good.