You can use paper towels at the bottom of your enclosure instead of substrate. The reason mesh is suggested for the top is a mantis needs a good surface to grip and hold onto while they’re molting. If you don’t provide them with the correct molting surface, you risk having them mismolt (falling, getting stuck in their old molt, etc). This is why I’d reccomend mesh and not paper towel. Mesh is easier for them to grip and hold onto while molting. The mesh I used is from a butterfly net from the dollar store, then I hot glued it to the top. Don’t use metal mesh!