It works fine on my thick AP cage. Just a thin UTH gives all the heat I need. It is completely outside the enclosure and I didn't have to drill a hole and figure out how to run the power cord through the wall.
I know the heating pad being suggested but do not know of one with a detachable cord. So you will either need to make a large hole for the plug to pass through or cut the cord and redo the plug to keep the hole small.
If you have vents you may be able to remove one and run the cord through the edge but that will depend on where they are and how they are attached. It seems like a good bit of work when you could simply put a UTH under it and be done.
This is the one I am running under that cage. It is super thin so it gives no rise. I just have the plastic connection sitting right up against the back of the cage so that it is not under any pressure.
https://www.amazon.com/Flukers-29052.../dp/B00164K2H0