+1 vote for the Lock 'n' Lock containers.
The 1.2L size (square footprint) will perfectly fit a Heat Sheet and you have plenty of room left over for other survival stuff. It's pretty bulky, but for sitting in a vehicle kit, no big deal. Plus you get a nice water resistant seal to keep everything dry.
Runs about $3-4 at Wal-Mart, near the Tupperware and such.