A good solid rack that totally surrounds the tub helps to keep heat up. I have built both the open style and the closed style in my hobby breeding center and the closed is the best for maintaining good gradients. You can go to Home depot or Lowes and get the 16" wide laminate shelf boards and build it pretty easily. Also I like to put the T-stat on the flex-Watt (I use flex Watt on all my tubs as it is cheap and easy) then put a thermometer probe on the bottom of the tub inside. Once you find a the temp of the UTH that creates a temp of 92 on the actual bottom of the tub maintain it for 24 hours and see if the bottom of the tub stays the same. In my multiple rack systems hooked to one T-stat I run the 24 hour test on all tubs and use the setting that makes the hottest of them all 92 degrees. This results in all the tubs being between 90-92 normally as long as all the UTH's are the same and the tubs are the same. We mist our tubs daily and while we are at it we use a heat gun to check the hot side temp and the temp of the snake. After getting it all set up we remove the thermometers as I still have not found a probe type thermometer that is 100% accurate and I have tried around 5 different ones, but my temp gun is an ISO-9000 calibrated piece of equipment that came with a certification sheet.