Oh I had misread I thought you meant you put a human heating pad inside of the tub.my mistake! I would suggest paper towels, or aspen inside of the tub. That seems to work very well. As for the hydrofarm, I just had one in my tub and I had to turn it up 6-8 degrees of what I wanted. as for the heating pad I have the same one, in a size small on a 16 quart tub.
What I did was I did not remove the paper backing and I taped it onto the bottom of the tub with aluminium foil tape. That keeps it on.
This is the guide that I used. It works wonderfully!
http://ball-pythons.net/forums/showthread.php?t=139156
Sent from my Galaxy S5