Well, the only thing with a gap that size is that you are loosing heat and humidity. You may have to boost up the thermostat to get it at the same temp. You may want to just leave the lids on, that will take up some of the gap and take care of the temp and humidity problem. When I made my rack, I designed it with the lids on, I wished I didn't. But I just cut the lid in half(more like a 1/3 of it) and then I duck taped the two pieces together on the outside of the lid and the tape acts like a hinge, so now I do not have to pull the tub all the way out. Or, like llovelace said, put a thin piece of board under it and put the Flexwatt on that.