The dimensions of the box are completely up to you, but I recommend sourcing the racks first and making the box to fit them. If you want to copy my design, with 30x40cm racks, here are the pieces you'll need.

12mm plywood:

• Two 32x43cm pieces for the top and bottom
• Two 32x88cm pieces for the sides
• Two 43x90cm pieces for the back and door
• Several 1.5x31cm pieces for rack holders
• Some pieces to mount the switches on

Other wood:

• Six roughly 2x2x30cm pieces for edge braces.
• Four chunks for legs.
• A 31x40 piece of pegboard, or any board with an array of holes like that

The top will need a hole cut for the fan. A computer case fan would work fine, but would require a 12V power supply. I used a 100V AC one because I happened to have one.

The back will need an air inlet hole near the bottom. Mine is an 8x10cm rectangle, but the shape is not critical.

Screw the edge braces onto the top and bottom pieces like in the picture. A scrap of plywood is useful for putting them in the right place.

Then screw all of the rack holder strips onto the side pieces before screwing the box together. The bottom set of holders is for the pegboard. Like this:

Your heater will not be the same as mine. It might be some high power light bulbs or a hair dryer heating element or something from a waffle iron, etc. As such, you'll have to figure this step out on your own. Just set things up so that the incoming air has to pass over the heater before moving up through the box. Once you get this together, the fan and switches are simple. Here's the circuit I used, but variations will work fine. One thing to note is that I put the heater switch and circuit after the main switch so that the heater can't be on when the fan is off. The thermostat is just a safety precaution that was conveniently already attached.

The door needs hinges and a latch. I chiseled out hinge recesses so the door will seal better. The latch was two small eye screws... Is that what they're called? Those screws with the closed circle on the end? Anyway, a stiff bit of wire was bent into a hook that holds the door tightly closed. Any latch that keeps the door snug against the box will work fine.

Be sure to clean out all the sawdust well and wash the racks. Then it's time to test things. Important: Don't wander too far during the first couple hours and check it often for hot areas developing on the box. You want dried food, not a burnt down house. If some part of the box is getting uncomfortably hot, you'll need to handle that before doing anything else. The fix may be as simple as a layer of foil over the wood to reflect the heat inward, but moving the heater away from any wood is a safer idea.