what kind of heat is being controlled by the probe that you're talking about? If it the uth, the probe should be between the uth and the enclosure. If it's for a heat lamp/che, i'd hang it down but not let it touch the ground. You can secure it with tape outside the terrarium.
In regards to the heat, bps can get some of their heat from belly heat but you want to make sure that their ambient temps are within acceptable range (around 78-83). Even with belly heat, low ambient temps can cause sickness.