It might not be effective enough but, you could try putting the pad on the side/back of the tank. takes the substrate out of the equation, but obviously side heating isn't as efficient.

If you are using a thermostat (which i highly recommend when using any heating pad for a live animal) you could put foam board insulation box over it to direct more heat into the side of the enclosure. If the thermostat is working properly and your probe is between the pad and the enclosure, it wouldn't get hot enough to be a risk to damage the heat pad.

if not radiant heat panel (aka RHP) might be the way to go. still recommend a thermostat with that as well. probe right on the panel, theyre a little more difficult to dial in because of the different heating element, so dont get discouraged when youre finding you gotta set the thermostat to an odd temperature so you have your 88 degree hot spot.