(Semi) newbie here with many, MANY questions!

[Snagrio]

You want a thin layer between the bottom of the tub and the UTH to allow air flow, and same for underneath the UTH and table/ surface.

For mine I have about 20 3-4 mm holes on each of the long sides of my tub. You can make more or less depending on how much you need to adjust the humidity.

To answer your earlier question on when you should switch from the 10 gallon to the 30 gallon tank, I would say that once your snakes length is equivalent to the sum of the length and width of the enclosure is when it’s a good time to switch.

How would I go about propping it up then? Just put some thin books or the like underneath the corners of the tub so it's just hovering above the mat, and some kind of heat-resistant object (like say a slate cutting board) under the mat itself?

EDIT: Just saw a video that most heat mats come with little pads to raise up the heat mat so that answers the second part of the question.