Home DIY

I'd go for:

1. Use a multipurpose drill bit (or a metal drill bit) to drill a hole through the steel that is wide enough to accept a:
   
2. Metal tap, which you run through the hole to thread it to accept
   
3. Some slim bolts
   
   

This is all very simple- if the plaster is ~2cm thick then I'd just use a metal drill-bit, make sure the drill itself is NOT on hammer, then drill the holes where you need them.

Turning a tap through a hole is simplicity itself- it's like turning a corkscrew,

Screw in, back it out, whack your bolts in, wallop.