If you're making the vivs from melamine, you can have the UTHs outside, underneath. I've been using that system for years and it works well.
You need to make sure the viv is raised slightly off the UTHs so that:
a) the weight of the viv isn't resting on the UTHs - this can cause overheating and damage the UTH
b) there's airflow over the surface of the UTH to prevent overheating.
It's especially important to use a thermostat with this kind of arrangement.
I raise the viv up by making two mats out of cork floor tiles, taped together to make mats the same size as the viv floor. I put one on top of the other, with a slot cut in the top mat the same size as the UTH, where I want the warm end to be. I then put the UTH into the slot and put the viv on top of that.
Sorta like this...
====================== Floor of viv
xxxxxxxxxxxxxxxx----------xx Top cork mat with UTH
xxxxxxxxxxxxxxxxxxxxxxxxxxxx Bottom cork mat
Not sure if that's very clear, but hope it helps. It's a useful setup to avoid having to put the UTH inside the viv, which is never ideal due to the risk of spills etc.