The zoo-med ones do say to use under glass only. They also say that their smallest "mini" one is for use with plastic even though the package also says to use with glass only. I agree with the idea to use a thermostat. I've done that successfully with the thermostat kept at 90º which seems to cool to melt plastic.
i doubt it will but it's probably safer if you don't use it on plastic glass aquariums are really cheap under 20$. BTW this should be posted in in Housing forum