I used a piece of polyethylene pipe (thick walled semi flexible black water pipe) Cut out slugs with a hollow core punch. Warmed up the slug and stuck it into the primer hole. If you warm it up enough, the PE will soften and ooze through the flash hole, so it won't come out.
It has a little bit of give, probably about the same as a hard primer. Silicone is much too soft, not much different than not using a snap cap at all.
I seated a jacketed bullet with shellac so it won't move, coloured the case with a marker.