If you have access, via library or local amateur radio club, to some of the 1990s and early 2000 editions of the Radio Amateur's Handbook there is a 28 vdc to 13.8 vdc converter project good for 10 or 20 amps. It's what I'll call old (but still valid, practical, useful) technology and modifying the design for reduced current is likely as simple as reducing the number of pass transistors. The project is described in detail though I would not say it's so simple anyone could do the project. In terms of performance (capacity, regulation, current limiting) I would say it's overkill but again, the article is detailed so it might be an option.
I cannot tell you if the components used are useful in the chamber. My concerns - at any significant amount of current (you'll be easily in that range) it would seem that failure modes could result in lots of smoke - maybe not fire but smoke. I don't know how well your chamber can clear something like that but I can tell you that many designs, including the one I've described, were not designed to be failure-proof or to fail in a safe (smoke free in this case) way.