Eric,

There are a couple of different materials you can use for shims. Hobie Cat sells a kit that has a number of brass strips that you hang on the upper edge of the pylon before re-assembling the boat. I've also used strips of vinyl house siding. If you cut them right, they come with their own "hanger" - just clip them on to the pylon and re-assemble. The vinyl deforms to better fill the gaps than the brass does.

Either way, you'll end up renewing the shims every couple of years. The only semi-permanent fix is epoxy. But even then, if you grease the pylons before re-assembly, it's still relatively easy to take the boat apart without resorting to a torch.

A word of advice - DO NOT use a carpenter's hammer directly on the castings. Use a wood block to spread out the blow, or better yet - a rubber mallet or dead-blow plastic hammer. Broken castings are easy to replace, but it's an aggravation you don't need.