A thermostat will not make your UTH get hotter. If the UTH doesn't get hot enough to be 90/93 on the surface of the glass as measured by a thermostat probe, then it's not big/powerful enough.

What the thermostat does is keep the UTH from being too hot, and it regulates the temp to stay at whatever you set it at.

As for placement, in a glass enclosure, I always recommend a UTH which takes up 2/3 of the bottom length of the tank. That way, you still have a cool side, but the UTH covers enough area to help keep the enclosure warm. Which is hard to do with an aquarium type enclosure. Place it all the way at one end so it covers 2/3 the tank.

The screen top can be virtually sealed off to hold heat and humidity in. Use duct tape, foil tape, cardboard, aluminum foil, plexiglas, whatever. Just cover all but a couple inches on on end for air circulation.

A heat emitting bulb is a great investment. I have one that I used for 2 years on a QT tank, which I now use on my scorpion viv. They cost a bit more but last at least a few years if not knocked around.