New Baby + UTH Question!

[ThatOneSpazz2]

Ok so, originally I was planning on waiting a bit to get another snake-- and it was preferably going to be a kingsnake morph of some kind.

...Then I found an absolutely gorgeous little baby Rosy Boa in need of a home; A San Jacinto Rosy Boa female, more specifically-- and next month I will officially be bringing her home!
(I'll gladly update with a pic or two of her when she gets here~)

So far my setup is going just fine! Got all the necessities ordered and things are already getting here. I'll have the full cage up and running smoothly by the time my little lady gets home, but here's the dilemma:

I know Rosys are burrowers. A good thick substrate is a must for them! She is a tiny baby, so she's starting in a little 10 gallon glass tank, with an appropriately sized, tiny heat mat.
Due to it being so small, I am unsure if I should mount it underneath like usually suggested, or on the side?
(I worry the heat won't reach through the thick bedding.)

To add more info: I WILL also be using a low-wattage basking light on the same side, to help with the temps and all that. Both of which are going to be regulated by a thermostat of course.
Should I just mount it on the underside anyway and let the basking light take care of the "above" temps, or mount it on the side??

Thanks for any helpful input~ !