Thanks Eric, I have three of them. One is to regulate my quarantine tub the other two are backups. My mine controller is a Herpstat 4 which I love. I just got the impression the OP was trying to keep costs down a bit and I noticed they drop the price on the VE-100's down to $85 recently which is a decent price for them.
I've always taped them to the bottom of the underside of the tub with the temp probe sandwiched between the UTH and the tub. As for setting it on top of another tub it should be fine, remember the point to controlling the UTH is to keep it around 90 degrees or so for the hot spot. I have dozens of tubs stored in my garage (not with snakes in them) where it gets well over a 100 in there in the summer months here in the carolinas.