What type of thermostat do you have for the warm side? If it's a high quality digital(at least a Johnson or Ranco), then you can get a Lutron Credenza dimmer from Home Depot/Lowe's, plug your COOL side flexwatt or whatever into the DIMMER, then plug the DIMMER into the THERMOSTAT(bolding so no confusion). Dial the dimmer down about 2/3-3/4 of the way, and the thermostat will generally keep it at that temperature(within 1-2 degrees or so). You shouldn't have to adjust the dimmer again(I said shouldn't, but just watch it).

I've done it this way quite a bit when I had a quarantine snake out in the living room or something during winter and one piece of flexwatt wasn't enough. The dimmers can take up to 300 watts, so you're not going to hurt it with one piece of flexwatt.

If you need a picture, I can take one.