If you just want something to support the UTH (keep it against the bottom of the glass tank), one thing I've used in the past is just a scrap piece of folded "hardware cloth" (welded wire mesh). I'm always making my own tank tops- no, not the kind I wear! And then I used a tile underneath it (much cheaper than a fancy silicone mat)- the tile wasn't thick enough to reach up against the UTH, & the "hardware cloth" has tons of air flow.
Setting up a tank, you can really get creative- there's many ways to do things that will work & be safe. I guess that's why if my garage could talk, it would tell you what a "pack-rat" I am. I rarely need to run to the hardware store.