[FigureLLC]

i would build it up on the outside like you said with 25ga steel 2x4 studs and use R15 fiberglass insulation with the kraft paper removed. i've read posts discouraging hardi board bc it soaks up heat and will take more time to get the oven to temp. and if you're using elements from only one kitchen oven, it'll probably already take a long time. i'd use tadpole seal around the door and a latch, both from mcmaster-carr. draw latches work great for a tight seal, but they're not ideal since you need a free hand to latch them. they have lots of different kinds though.