You can get an Accu-Rite Indoor/Outdoor thermometer that will also tell humidity at Lowes or Wal-mart for about $12 each. It has a probe that you can run over to the warm hide and I set the unit near the cool hide and run the wire under the substrate. At Wal-mart they are located in the Hardware section. At Lowes I found them in the Outdoor/ Lawn/ Garden section...
Also if you have problems with the temp/ humidity, let me know and I'll post the pics of what I did to my screen top that worked wonders