Here's a little box I made from walnut and horn. The "lens" on the lid was also made from horn that was flattened. I cut the round disk, polished the inside surface and glued in into place (I know, I know - probably not PC). The outside sat proud of the lid and I finished turning and polishing it on the lathe. The compass is quite visible through the horn lens and the lid easily removes for a better view. I'd like to have made the lens thinner but, being curved, it's pretty thin at the edges.