I also built a ferm chamber of pink insulation and plywood under my workbench in the garage. I picked up a used window AC unit off CL for $40, cut a hole in my ferm chamber wall, and mounted the AC unit.
The AC unit is connected to a Johnson digital controller, with the probe taped to the side of my carboy. I had to reroute the AC's internal temperature sensor from the front grill out the back of the unit so the compressor wouldnt kick off when it sensed that the ambient temp of the chamber was getting cool, but that was super easy. Just popped the grill off, unclipped the probe, and fed it back through the control panel and out the back of the unit.
It keeps perfect ferm temps even in my garage in oklahoma summer weather! Much cheaper/easier than chopping up your freezer I'd think...
Heres some photos. Note: I do not claim to be an expert wood-worker by any means. The water stains on the floor are from a previous incarnation of this unit. Always make sure your AC unit is angled so it drains out the BACK