If it's just to use up a sample block I might just use left over batting. For mats for hot pots to rest on I would probably use an insulated product.
I can feed my addiction at the Fabric Depot here in Portland. (For those of you who are not from the Pacific NW...Fabric Depot is 1 1/2 acres of fabric, bridal, home dec, needle craft and more of heavenly things. They do have an on line site.)