I'd imagine hot-glue would also be an easy way to fill in the primer pocket. Seems like it'd have closer consistency to the acrylic/whatever that the "official" snap caps have. It'd be real easy to repair/refill as well (and you could easily melt it out to reuse the brass as it was intended)
"I don't believe that the men of the distant past were any wiser than we are today. But it does seem that their science and technology were able to accomplish much grander things."
-- Alex Rosewater