I have a Coleman 70 Quart Xtreme cooler that I've modified for my mash tun. I seem to be losing a lot of heat out the top. Are there any good ways to insulate it to help prevent heat loss?
A towel on top helps prevent a lot of the heat loss. A plastic cooler, by itself, is pretty good insulation, though. In a single-infusion mash, you should only be losing a couple of degrees in an hour.
You might want to test for leaks around the hinges. I had some warping on a cooler there that I used to bring to the beech. Pour some water in it, go outside, flip it over and hold the top on tight. If there are leaks, then you will probably want to put some foam adhesive strip around the lip of the cooler. Something like Scotch Foam Tape.
But Nick and swgill definitely have the simplest solution. And it always handy to have a towel. :)
I'm thinking you must have some sort of leak or crack in your cooler. I use one of those orange home depot coolers and it maintains a very consistent temp while mashing. Maybe try filling it up with hot water and see if the outside springs any leaks. If the top doesn't fit properly, maybe try a layer of that spongy tape along the rim and a weight on top.
The cooler I use is this one, btw. You can unscrew off the spigot and screw in a valve really easily as well.