There's a couple of different ways to do it.
You can use a slightly larger pin, but the problem is that the holes in the castings aren't evenly worn. Unless you drill out the castings (both the lower rudder housing and the one-piece gudgeon), you're still going to have slop.
A less drastic solution is to fill the holes with epoxy (sand the holes first to get really good adhesion), then drill them out to 3/8" again. You'll end up with a thin layer of epoxy around the hole and virtually no slop for about a year.
A more drastic solution is to drill out the castings to accept plastic bushings (Hobie Catalog page 32
here). Long-term, that's probably the best solution, since the bushings are easily replaceable if they get worn.