1) It would be able to maintain temperature for the duration of the process.
2) It would not leak, for a steel tank with a steel lid, this is actually easy, put a neoprene ring in the lid, so that when the lid and top of the tank meet, the neoprene fills any voids.
3) Would have a plastic top cap, on a spring over the filler hole. When filling and emptying the tank, you hold the cap open, otherwise the spring holds it closed. If pressure builds, the pressure pushes the cap up, and releases the pressure. We add a small weighted latch, so that when the tank is not sitting flat, the cap is held closed. You can still use inversion agitation, during the 5-10 seconds your agitating the tank, pressure isn't going to build enough, that during the 50 seconds it's sitting flat, it can't be released. Of course when pressure is released, you might burp a little chemical, but you can wipe that off with some paper towel.
The fixing process, could change the temperature slightly, fixing, is not that critical temperature wise, it's development where temperatures are critical.