I've never thought about installing an extinguisher in the cabin so I have nothing to offer on that. My car does have one mounted in the trunk by a PO.
You do want an all metal bracket and extinguish . No plastic. We usually used AN 525R10 screws with nylon or metal lock nuts. Never a sheet metal screw.
Charlton's install looks good. A belt attach point is very strong . Probably the best you can do considering the limited space. Ask him for the bracket details.