Finman,
Square, rectangular or circular cross section?
Jezd,
I would think a square or rectangular tank of 'just a little larger' than the box dimensions would be best. Easy enough to fabricate (and insulate), really.
Need a grid to be able to submerge frames, if put in loose, and a lid to retain the heat (and safety), ideally. A properly designed and constructed stand (from scrap pieces of angle iron?) would be far safer than 'a couple of blocks'. A gate valve, or similar, for draining the tank would be good. An open wood fire would not be recommended for indor installations.
Just a few thoughts to be going on with.
Regards, RAB