Should be a spring around that port that your cap is covering and a bolt through it compressing the spring. Bet you can find those two parts super easy in a salvage yard for little of nothing.
I am very familiar with that particular cap. When I did some modifications to my engine, i needed to use a bunch of caps. Some of the caps were thick, black rubber and I bought them in individual sizes. I also ended up buying some of those type of caps because I couldn't find the black ones in the right size at the particular store I was at. The black rubber ones are still doing fine. All of the ones like the one in your picture failed within 6 months. You are going to need to do something about that cap pretty soon.