You could make sure that the noggins are large enough in width so that they protrude beyond the profile of the RSJ and screw the board into these from the underneath, or you could fix the plaster board using the dot and dab method.
This is how I have done it on many jobs in the past.
Hi worm, Masterplaster solution are good. You could also, (a little more complicated) build 2 timber frame studs to go the full length of both side of RSJ.
So you can securely fix timber to ceiling and wall at both ends, this should drop just below bottom Of RSJ. Then board over that, remembering that RSJs should be either double boarded of fire boarded. Fire Regs.
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