We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember The Other Mike
saying something like:

So I'm thinking of lining it out with ply and filling the walls and
ceiling with some rockwool. Could I just batten it out with 2x2
fixed with a grab adhesive and then screw plywood to it or will I need
a vapour barrier behind the ply?

Not rockwool or fibreglass against a steel structure - permanently wet,
even if you install a vapour barrier, it would need to be perfect.
I'd spray it with foam, as suggested.