Why not try putting it in his hide on top of the substrate? I secure the two probe wires of my t-stat along the wall of the tub(with clear packing tape-isn't very sticky regardless so no harm done) instead of taping it to the bottom and then run the probes themselves into the warm and cool hides. They maintain the hide temps very well and the ambient temps are just controlled by the air in the room. Amani thermoregulates quite normally and spends an equal amount of time in both hides. The temps inside the hides measure 95 and 84 respectively. I haven't had any problems with fluctuations such as when she lays on the t-stat probes(ok, maybe the cool side fluctuates from 84.4 to 84.5.. jeez, LoL).

Just my two pennies