|I'd just frame it with sawn 4x4, tack expanded metal lath |on the sides for plastering, and line it with a bit of planed timber. |Unless there's a lot of brick sitting on top of it as well as the |joists, in which case it will probably need steel.
If you use planed 4x4 you wont need much in the way of framing, nowt but a strip of 3/4 x 1.5 or so.
NT