In Need of Dope Minds! Cheap Temperature Control. - DYI

Ok, so before I take a crack at this I would like to run it across the experience of this site. Basically my situation is cold nights dropping to 35 degrees for the next month or so. My setup is a well ventilated cabinet.

Things to note:

- I am looking for a DIY cheap solution.
- Working with a $50 budget.
- I have power tools & tools.
- I am in the mood to rig some stuff.

My first thought is to get a heating pad from walmart and a cheap thermometer. Right now the plants are getting to cold and water is not drying properly.

I appreciate any feedback and ideas you have for me. Here are some photos of the cabinet. The light is a 2 foot dual t-5 flouro. Thanks in advance.

Some kind of heating implement hooked up to a rheostat.

I went to lowes and got a space heater w/ built in thermostat for 25 bucks (actually I got the floor model for 15), my temps were getting in the 40's...I could keep the box in the 70's w/o the exhaust fans on, but I knew they needed to breath. with the heater on 75 it's stays 75 now