I've never heard of a burrowing BP. For lots of people the only substrate is a few sheets of newspaper. If your heat mat is so far away from the surface of the substrate that it cannot get it up to ~90f, you need to remove alot of that substrate. A heat mat (in my experience, yours may vary so check) won't get hot enough to burn a snake. If you just plug in the heatmat without a thermostat and let it get as hot as it can get before shooting a temp gun at it, you'll see it reaches a max of about 110f - about 44c. The minimum temp for causing a burn to human skin (after 5-6 hours of contact) is 44c. Snake scales are far more resiliant than human skin, besides, BPs can see heat and they aren't stupid.

If you start mounting substrate on one side to correct the temps, it's going to drive you crazy because your snake doesn't care and will mess it up all the time. I reccomend just a heat lamp on a dimming thermostat to regulate the ambient temp and a heat mat for the warm hide, also on a thermostat. Don't worry about having tonnes of substrate, 3 sheets of newspaper laid flat is fine. The point of substrate is mainly to soak up urine. The amount you need is the amount it takes to soak up a pee.