I don't have the rock-guard on mine, but I do have a misc file I've saved of tips from online forums, and I have this in it:
C16-24176 Gas Spring.
I hope this is the right one. You can find a lot of different "strengths" of the same basic shock, on Amazon or eBay........as I recall, something on the order of 15 psi is about right. You don't want to go a whole lot heavier-duty than that, or it makes it hard to close.
"You'd care less what people think of you if you knew how seldom they did." E Roosevelt