KSA- yes, bcr229 beat me to answering. And I 'second' the info on heat mats designed to handle moisture that are made for other animals. I used to have
one (made for whelping quarters etc) that was about 16" x 24" long...that was a "small" one. Can't recall the company that made that one...I've also seen
those for livestock referred to as "pig blankets", most are quite large. You could also check with feed stores, they carry such things or could order them.