Sounds like rbeech uses something like I do. It's like a 20"by 48" plastic place mat. I swiped it from my wife's gardening set up. I used to use a thermostat sans/temp- scale so I had to fidget around to find the correct setting. That said, it's easy enough w/out the thermostat. Watch the thermometers in your developing trays and just plug it in a minute or two every 15-20 mins. It depends on the temp of your room I guess. I've tried it underneath a water bath tray to smooth temp swings but found that not necessary (for me), even with 55 degree room temps.