Not to be a bummer, but I am looking at the PDF of my UTH manual online right now:
"The ReptiTherm® is for use on standard glass aquariums only. We cannot be responsible for
custom or homemade cages and use of the ReptiTherm® on such cages will not be covered by
the warranty. Do not use the ReptiTherm® on anything other than glass terrariums:
NO ACRYLIC cages
NO METAL cages
NO WOOD cages
NO PLASTIC cages
(Only the RH-7 Mini is suitable for use on “Keeper” type plastic cages
because of it’s low heat output. Always place the heater on the back or side wall of this
type of cage) " - from the PDF
Which means that only the mini size can be used in the plastic cages and you'd have to install it on the side of the wall, which would defeat the purpose to providing belly heat. The reason is that the heat will dissipate more with side mounting and reduce the risk of melting. Putting a UTH on the bottom of a glass terrarium also requires the elevation of the enclosure to allow air flow beneath the UTH to prevent burn out, but the glass is more insulating and can handle increased temperature gradients. I don't trust the store personal to read the fine print and you should look for another solution, sorry that everyone seems harsh today, but they are right this time.
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0.1 Albino Leo Gecko
0.1 Crested Gecko
1.0 Dear Boyfriend
Departed: Harvey and Spock