I have already made this for the two very large litter boxes I use. I recreated this in a smaller version for this instructable.
Step 1: Items Needed:
Litter box -- for this type of project you'd probably want to use something much bigger like an under-bed plastic storage case. The one I have is 2x3 feet. (not the one pictured)
Step 2: Measure the inside bottom dimensions
Step 3: Measure and cut the mesh.
Cut the mesh. YOU WILL GET CUT HERE! Those little prongs are very sharp and you will invariably get scraped or cut. Be very careful!