is the bottom going to be melamine?
I wouldnt put a
UTH directly under a melamine cage, depending on the thickness, the heat required to warm the floor of the cage may end up burning the bottom of the cage itself.
The size sounds good, albiet maybe a little taller than necessary, but thats really completely up to you. It wont hurt being that tall, its just going to take up more space and cost more to build.
But on the plus side, if your going to make it that tall, you may want to cut out a basking vent with some type of soft screen or hardware cloth and use a basking lamp in lieu of a
UTH.
Or try one of those ABS plastic heat mats , or something like that.