I filled my cooler-tun lid with Great-Stuff. My lid just happened to have holes for shooting the foam in. Because coolers are usually build to keep things cool, their bottoms and sides are very well insulated. But using a cooler as a mash tun, tons (pardon the pun) of heat will pour "up" through the lid. So insulating the lid helps tremendously. Great-Stuff worked fine. I only put in about a 3/4 inch layer in my lid, and my mash temp doesn't move for 1-1/2 hours. It took a LONG time for great stuff to cure though. I suggest extra ventalation holes, plenty of time (2-3 days) and loads of humidity. I filled my MT/cooler with nice hot sparge temp water, and popped the lid on to the humidity would seep through and cure the foam. Before I did that though, even after 2 days the foam was very goopy in the core of the lid. I've seen great-stuff pull 16 penny nails out of walls, bend 2x4's, etc... so please give plenty of room for the stuff to expand.