The commercial UTH WILL get very hot. There is no reason to abandon the UTH, especially for a 20g tank. The UTH will work just fine. Now, if it's not getting warm enough, then there may be something wrong with your UTH. Try plugging it into the outlet unregulated; it should get well over a 100º (obviously don't do this with it still attached to the tank!). If it's not then it might be defective.
You should be able to get it warm enough with just the UTH and all the insulation that you have. And a t-stat can definitely do heat lamps. If you wanted to be safe get the Herpstat II, I have the Pro and absolutely love it. I put the probe on the warm side directly on the glass under the substrate.