Hook it on the leading edge rod and it pops right on with a little force. Another Rudy Merz design. He understood that it was going to be necessary for the shooter to remove the cover, so, he designed one that is tool-less. Adjustment is achieved by slightly bending it as needed, although can't recall ever having to do this, but, with a repo part it might be.
Parts for the 118 have always been essentially impossible to find. Never saw a single 118 part in any Crosman repair station.
Good luck on the rebuild. Great gun.