Hey there! LOVE the "Hot Glue Placemats"!

Drink coasters need to be absorbent so consider using an absorbent base for the hot glue...i.e. cork sheeting, thick felt, possibly PUL fabric (the fabric used in cloth diaper covers) for water resistant backing but try the glue on an inconspicuous spot on the fabric as I have not tried heating PUL fabric. Also, hot glue can get brittle if cold or melty if very hot. Interested in finding out if your experiment works! :)

I don't see why not.

There is a waterproof fabric by Roclon that I get at Walmart or JOANN'S......it is used for curtain lining and it is totally machine washable & machine dry. I use it for bib backing, waterproof wheelchair cushions etc. So I'm sure it would work for coaster backing too.