Yeah I do that. You can get different thickness from Home Depot. I use the white stuff located in the lumber dept. It came with a thin layer of plastic attached to it that said mold resistant or something similar. Can't quite remember. Mine doesn't come in contact with the mash although the corner hits occasionally when I am putting it in or out. I think most coolers taper a bit or have a lip just inside the top that it can rest on. The real handy deal is you can poke the thermometer right through it and it will hold it in place. Really holds in the heat.
Next Brew: Cascades Orange Pale Ale
Primary: Indus IPA
Secondary: Belhaven 80 Shilling
Carbing Up: Simply Hefe
On Tap: Fat Tire, Watneys Cream Stout, Bombshell Blonde