im having the same issue, i have a probe on the floor of my sterilite bin, and the thermometer/hygrometer on the other side. and under the substrate is 95F, (not sure what it is on the surface... using a 1" thick bark substrate) and the thermometer is reading 72F on the other side, which is only 2 degrees warmer than room temp, with a humidity of 60%

my UTH covers about 1/3 of the bottom, and is hooked to a reostat. (currently at about medium/high) Its been sitting like that for almost 24 hours now..

so ill be experimenting with ways to keep heat in, and ill let you know if i find a way.