Just thought I would share my little setup for my overhead heat and herpstat probe mount.
I have a fairly large piece of lexan underneath the substrate and I drilled holes in pairs so I could zip-tie the prob cord and the Plastic elbow (that has three holes and flat base found in plumbing section of hardware store). Drilled a hole out the back of the cage.
I am using a CHE in a cage which OP and the Serpant Merchant with a few others helped out with converting over from Beardie use to BP.
She hasn't attempted to lie on it and can't knock it over because it is zip-tied. This setup would require loose substrate though I think. This is also a 2' tall cage. My failsafe is the wattage of the CHE though as at 100% in this larger cage it won't raise the temps high enough to become too crazy. I think with smaller PVC cages that lack the height this may be a larger issue and a second t-stat failsafe would become more important.
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Not sure if it helps but couldn't hurt. I had the thin lexan piece laying around but the pipe pieces cost about $2.50 total.