I have read, even in newer snake books (one specifically I'm thinking of, pardon me I don't have it with me right now but it was written in 2012/2013 by a veterinarian) have suggested putting UTH on the bottom or side of tanks as well.
I think their primary function is to be used as a source of ground/belly heat for which to digest food properly. If it is on the side of the glass, it may heat up that general area of the enclosure enough for proper digestion (above 75F minimally is the required temp for digestion for corns, I believe).
I agree though that it might work best on the bottom, where most have put their UTH. What is the ambient temp that worries you may be too cool?
Unless it is not the ambient temp at all. Perhaps your enclosure is made of wood or some other material in which glass is only on one side and not the bottom?