Tubs are fantastic, and I'm definitely not ever switching back to tanks.

I just drill the holes near the top of the tub all along the sides.

For the UTH, I just get those ZooMed UTHs and they do just fine. I do recommend you get a thermostat. Try a ReptiTemp 500R, cheapest one you can get http://www.reptilesupply.com/product.php?products_id=75

Does the digital thermometer have a probe? If it doesn't, try buying one with a probe. Put it on the bottom of the tub right above the UTH and that should accurately tell your temps.

Aspen is fine to use, it's what I use as well. Most people like using paper towels or newspaper but I like loose bedding a lot more. Just easier spot cleaning. You can try out the newspaper or paper towels and see how that goes for you, or you can try other stuff like cypress mulch.