What kind of heating pad should i buy?

[PitOnTheProwl]

Placing it's easy. Peel off the paper backing, stick it onto the bottom of the tub on one end - easy peasy. I sandwich the t-stat probe between the pad and the tub.

(Also, I went to look - I'm using ZooMed, too.)

Even easier to remove the wax paper and cover it with aluminum foil tape (sticky side to sticky side) so there is no adhesion able to stick anywhere.
Now you just use foil tape along the edges to secure it to whatever you want AND its reusable without risk of damage.