I built this oven a while back, but I couldn't get it over about 275 degrees. I recently added a layer of R13 insulation to the inside, and now it gets up to 400 no problem. It takes about 30 mins to get there, so a total cure takes 50 mins. I did one test part (an old water pump impeller) and it came out great.
I used the control circuitry from an old microwave/convection oven to turn on and off a 3 element quartz heater (about 1600 watts). I just set the temp for 400, the time for 50 minutes, and it beeps when it's done! (and turns off)
I still have a few kinks to work out (like a structure for the ceiling to hang things from), and I need to get better at the application of the powder, but at least I can get up to and hold 400 degrees.