I built one using a cheap gun locker from Academy ($80). The heating element is a BBQ starter element from Amazon, has a thermostat like an electric skillet. Lined the inside with an insulation from amazon called turbo wrap.
it has a heavy foil on one side and a short wool insulation on the other. Mfg claims it is good to 1500 Deg. It has held up well. Takes about an hour but it will hit 400 deg. I thought about a fan but it is not necessary. I have mine sitting on an elevated furniture dolly, wheel it outside when i'm ready to spray. I like to preheat my parts to about 100 deg and then let them cool just a bit before I coat.
I have been using Cerakote and KG Gunkote. By the way ...KG has a color that duplicates gun bluing so well it is almost impossible to tell the difference.