Black or maybe
brown construction paper works great.
As you have noticed, temperatures drop off rather quickly the further up you go from the UTH. (I even noticed in some notes I have above my laptop that some GU member bought an 11 x 23 inch Ultratherm for his 36 x 18 x 18 in October 2014. The measured width of the tank includes the black frame and is not the actual measurement of the glass base.
You want temps from 88-91ish on the ground under the warm dry hide.
What's the room temp right now? To increase the warm air temps, I'd buy a 10 inch diameter dome fixture (no smaller) and either a 60 watt or maybe 100 watt CHE. That will warm the air. A Lutron dimmer/rheostat will tweak the output of the CHE to reasonable levels. (Sorry, I know, more $$.)
Some places like Petco will let you exchange the bulbs till you get it right. I've had problems with Exo Terra CHEs.
I turn off my leo's heat at night, but the room lows drop no lower than 65 F. That's not a problem as long as the daytime temps are good.
Different hides might hold the heat better. This medium Exo Terra hide costs about $5 from Amazon. Click:
http://www.amazon.com/Exo-Terra-Rep...584610&sr=1-1&keywords=exo+terra+reptile+hide