Having trouble heating my RTB enclosure

[aliamo]

So i am a newbie with an adult Red Tail Boa. I bought her with the enclosure she was being kept in which is a glass tank with a screen lid (dimensions are 48 x 16 x 16). I know she needs something different so I'm planning to purchase an Animal Plastics cage soon, however in the meantime I need some help getting my tank at the ideal temperatures.

I put in an under tank heating pad however which did not help with the ambient temp. So I got a 100 watt red heating bulb which I placed on top of the screen, however it seems to be making the tank too hot! I turned it on and a little while later the thermometer was reading 104! I have the probe for the thermometer on top of the substrate, right under where the light is. Is this placement correct? I only have one thermostat right now, but it is connected to the UTH, however I'm assuming I should use it with the heating bulb instead for now.

For these type of tanks, do people typically use both a UTH and a heating bulb? Or would just using the bulb alone be ok? I've had her for about a week now and have had issues with the tank getting too cool and now too hot so I'm worried I may be stressing her out! Any advice is much appreciated!