Do you use a filter on your lens? I have a b70 external cap I could part with. It bayonets on the outside of the lens instead of the inside. It will not work with a filter.
I just tried to fit the cap on my 50mm cfi. It fits so tight you can hardly get it off. I don't think you would be happy with it. Sorry, I should have tried it first. It was made for the 110mm f2 lens.