It's pretty straight forward really, just 16mm thick MDF supported with 45mm pine on the insides and 45mm aluminium rectangular bar on the front. The support is important as the mdf will sag without it. I then sandwiched fibreglass insulation in between the pine supports, and the inside skin is 6mm fibreboard for even better heat insulation.

Other than that, it's simply a 2kw heating element, an oven fan and motor (both purchased off ebay), and a shroud to force the air to circulate around the oven. You also need to mount the fan so that the fan is inside the oven, and the motor is outside. If you buy an oven fan rather than any other sort of fan then they are made for this and have an extended shaft to go through the wall.

Its really very easy, don't over think it. It's just basic woodworking to build a box, and you just mount a heating element and fan on the back. It's a glorified hot box. The hardest part is working out the doors and a way to secure them closed, even the wiring is easy.

Hapoy to answer any specific questions you've got!