Those circular thermometers sold in stores are CRAP for accuracy in my experience.
Placing them on the side of the tank will give you the temp for a snake living sideways. Not a snake living on the ground. Even when using lights, sticking them to the side is pointless.
With a UTH you (never use the sticky stuff for one) place it under a tank. You can lift the tank up slightly with bottle caps or furniture feet caps...then turn it on. You use a probe. or the circular one if you must, placed directly on the "hot spot" inside the tank.
P.S. heating pads or heat tape generally have no problem heating up through custom enclosures. But all of these items (heat pad or tape) should ALWAYS be controlled with a thermastat or rheostat.