I converted a chest freezer into a fridge by adding a temperature controller. It uses about 90-95% less electricity than a regular upright refrigerator. I converted it for less than $20.
Here's a link (they use a much more expensive controller)
This printed part allows me to do it without making any permanent changes to the freezer, (besides the two holes in the lid where it mounts). The fridge plugs into the outlet at the back of the box, and in the front is the digital display.
It uses the STC-1000 digital temperature controller, I bought mine from ebay, now they are about $10-15 USD.
The outlet in the rear is this: ($1 USD)
Cooper Wiring Devices 125-Volt 15-Amp White Duplex Electrical Outlet
I've used this for years without issue, so the more expensive controller is not necessary. If you're like me though, you may want to keep a spare controller in a baggie taped to the back of the freezer. In case there's a problem, $10 spent on an extra controller is a lot cheaper than the cost of your food going bad because you're waiting for a replacement controller to arrive in the mail. :)
I posted the Sketchup files, unfortunately I wasn't using OpenSCAD when I designed this.
After printing the enclosure and the lid, install the temp controller, and the outlet.
You may need to wire the outlet before installing it. If so, remember to run all the wires through the correct holes before connecting them.
I mounted mine in the right rear corner of the lid of my freezer (now a refrigerator). I can easily see when it's working, and if it has power.