Here's a quick pic-based wrapup of how we built a warmbox to keep kefir and sprouted brown rice warm during the winter.
We started with a steel rack so we didn't have to build a box. First, we removed the upper shelf and inverted it so we didn't have to work around the internal bolts when making our measurements. Next, we cut the upper and lower sheets of rigid foam insulation.
Inserted the back first, then the sides.
Figure out what wires you need and where you want them, and route them before you attach the sides and back panels! If you don't, it's not the end of the world.
The base is just a sheet of plywood inside a heavy garbage bag.
We were going to do something fancy with the temperature sensor but this worked just fine instead. I recommend having the sensor in direct sight of the heat source--otherwise it will be getting indirect heat while your warmed objects are getting direct heat.
Behind the lips of the top and bottom shelves we inserted a length of wood. The one on the bottom shelf holds two L-hooks where the cover sits, as well as four webbing straps.
The top one just has four velcro squares stapled on for the straps to attach to.
The cover is a length of the rigid insulation covered with corrugated plastic.
We got some soft foam windblock material at the 100 yen store.
The foam insert allows the 2nd chamber to maintain a lower temperature via passive heat. That small gap allows it to stay at 18 degrees or so with the main chamber set to 27 degrees, with an ambient temperature of 8-15 degrees.
That's it! The temperature controller is an STC-1000, which is inexpensive and can be sourced easily online.