If the Oak galls were fresh green ones, then a bone needle and thread would pass through them. If they were old, brown and dry, then there is already one hole partly through them. Cord and resin into the hole could be used to secure them.
If you were using thin walled mussel shells or perhaps cockle shells then a bone punch against a clay support works to an extent. The structure of the shell tends to limit the distance that a fracture will travel. So in my limited experiments I got several holes that weren't particularly round but they were servicable as holes and the fractures hadn't traveled away from the hole to the edge.