Hanging an egg chair between two joists
Does the plate have to be metal?
We recently did just this, with a piece of 2x8 bracing two joists, and 4 lag screws into the joists. A 2x6 is also fine. The screws must be 3in long as they go through the plate, through the drywall, and then 1 inch into the joist. Any wiring through or around the joist should -by code- be 1.25in in from the edge. You can use 3/8in or 1/2in diameter screws.
See also the &quot;cleat&quot; answer by Isherwood for Hang indoor swing chair BETWEEN two ceiling joists
Pre-drill the joist for the lag screws, but first use a 3/4in to drill through the ceiling drywall and confirm location of the joist to make sure you pre-drill in the centre of joist edge. I don't find stud finders accurate enough to drill blindly. The brace will cover these holes.
Optionally stain the brace.
Attach the chair's hanging hardware to the brace (U ring etc...)
As for the metal option, I'd have to defer, because the critical point is the welding of the U ring. You'd have to make sure that whoever welds it knows what it's for, and is aware of the dynamic loading. Perhaps it is better screwed or bolted onto the brace. If bolted, the nut can be recessed into the drywall.