Marine varnish is the most waterproof thing we've found, so far!

You'll have to apply several coats, and MAKE SURE you don't leave any uncovered gaps, no matter how small.  If there's a hole or crack where moisture can enter, it will.

It sounds like a great project.  Post here if you set up a gallery to show your work, so we can all have a look!

Welcome to the board, and when you have a bit of time, tell us how you made them.

Sue