Jerwald

Hello everyone I'm a new member on this site and looking to get into knife making on a small scale, just as a hobby. I'm trying to build a small gas forge for heat treating so I've been browsing and researching a fair bit. Attached is a picture of a shell of a forge that I'd like to make. It's out of galvanized ducting which I've read might not be a good idea. Any thoughts on that? Also I'm not quite sure what to do about the lining. I was intending to go the plaster-of-paris/sand way but I've read a fair number of comments saying "don't" so I'm looking for advice. Im trying to build a portable forge as I don't have an area where I can set up something permanent. Thanks in advance, looking forward to responses Jerwald