I noticed that you have a screen top on your viv. I also have screen topped vivs. We have been using heat lamps (from above) as our only form of heat until just recently. So long as the screen material is metal, you can put your lamp directly on the screen top with out a fire hazard...I have done this with all of mine.
Then what I have done to keep temps up, is to use cardboard on the top to keep as much heat in as possible and keep temps stable. To get it set up I turn my lamp off and then trace it on my cardboard. I cut out the area I traced, but just slightly larger. Therefor keeping the cardboard from touching the lamp when it heats up.
All of our tanks have been heated this way for the last two years, with no problems and it has kept temps very stable.
In having said this, yes a UTH and thermostat/rheostat should always be used as corns need belly heat to digest and thermoregulate. We started out with Garter snakes and that is part of why we did not have UTH's. The other reason we did not have them until recently was also because i knew that UTH's were well known for overheating and/or failing which can cause severe burns. What I did not know at the time was that they could be used with thermostats/rheostats to make them safe to use.