Yeah you're definitely going to want a t-stat to control the UTH. Yeah, it's kind of an investment but it's one of the best investments you can make towards the health and safety of your pet.

As far as the placement of the temperature probe goes, you should have it over where the UTH is located, it may take a little trial and error to figure out the best way to get an accurate reading. I usually have a layer of substrate with the probe right on top, you can try putting inside the warm side hide also.