Having your tstat probe on top of the substrate is a bad idea. Your snake can easily move it making the uth go to full power and possibly harming your snake. The best place for it is between the uth and the bottom of the cage. Having it outside leaves no chance for the snake to move it.

Hanging the probe is probably the best way for you to get the best results. I think you might get better results with a bulb. A che is directed heat so if you choose to use it make sue the probe is hanging in the beam of the che. A bulb will spill the light and heat into the cage and more evenly heat the entire area not just one spot like a che.