What else can I do to increase tank heat?

Ok I'm going to pick up a proper uth today see if that does the trick as I'm not currently using one. Hopefully that will do the trick.

Do you use any particular kind of thermostat to regulate the uth temp?

I use Herpstat. I have two Herpstat 1, a Herpstat 2, and a Herpstat 4. They are great.

A uth does nothing for your air temp so that will not be a fix. It just provides a good hotspot. Plus any good tstat will need to be ordered online as most pet shops only carry junky options.