I would not put a heat lamp on the tubs at all.

Long time ago, i had just moved cross country and did something similar as I was saving to get a better caging setup. My roomie's brother nudged the lamp and the burning plastic suffocated my kingsnake. I still feel terrible about it, even though he had been warned not to touch the tubs and still wanted to put his feet up on the table they were on. =_=;