Most UTHs will tell you to stick it to glass only, but what I did was use the little rubber raisers and put them on the paper side (the side you can rip off to reveal the sticky side). and then I just place it underneath.
OR
(By the way, heating pads are usually $25-$35 each so that may be expensive)
You can use a piece of plexiglass or glass and stick the heater onto that and put your Terrarium on the glass, locating the UTH where you want it.
ORRRRRR
You can buy heat tape or the heat-wire. Heating wire will allow you to move whatever you need, and put it wherever you want. Heat wire would be my reccommendation, but heat tape would also be a good choice.
numerous amounts of UTH can get pretty expensive
