Just finished another Coptic book with wooden covers. This one is made from quarter sawn cherry wood, with a piece of mica inset into the cover.
I used an old time finish on the wood (thanks to Jim Croft, who brewed it up). It's made of beeswax, pine pitch, and raw linseed oil. I just rubbed it in with my fingers, then buffed it with a soft cloth.
When I got the piece of wood, one edge was rough (on the right side in the photo). I decided to leave it that way when I cut the book covers.