Yes, now that you mention it, I recall others having trouble with ZooMed UTH as well. I tried one a LONG time ago & didn't like it, but I didn't use it long enough
to have any safety problems with it. Ultratherm has a much better reputation.
I'm glad CALM mentioned the foil tape....I wasn't even thinking about that, but that's a good option too. I actually have a roll of it (for other purposes).