In my experience it's the cap that fails on Renix XJ's. It hits the hood when the bottle expands and the threads strip out.
There is a Stant brand green cap made for mid-80s Volvos. Here's a quote from a "Joe Peters" on another forum:
Use a Volvo 740 2.3 L engine cap on the pressure bottle, Stant part number 10244. It will seal tightly on the bottle, OEM or aftermarket, it sits lower so it won't impact the underside of the hood when the pressure bottle expands and that will reduce cracking in the pressure bottle, and lastly it has a higher pressure rating and will raise the boiling point of the coolant.
I got mine from Advanced Auto for maybe 10 bucks.
You could look through a junkyard for a metal pressure bottle. I know there were some larger Mercury or other car that used them into the mid-90s.