Being cheap isn't for everyone, but it you are, here's a method that works fine. Bring your PS cap into the auto parts store and get a rubber hose that fits over the ******. Buy about a foot's worth. Stick the end in the hole in the plastic inner fender well. The cap won't get lost because it can't untwist against the rubber hose, which is wedged in the fender well hole. Not car show material, but effective.
OK, should be a hose clamp. But when I did it, that's what I had and haven't changed it since.
End of tube inside wheel well.
Yes, XGTZs.