Hey David, I'd probably make a pokerchip-like piece to fit in the hole, from either wood or plastic, or the two of them laminated together. What I decided to use would probably be most informed or influenced by what the bass was. For instance, I might try to make something a direct contrast, but matching something else... if it's a maple top bass with purpleheart laminates in the neck, and a mahogany core (a common combination) I'd choose the mahogany or purpleheart for the chip. Maybe both, in some decorative pattern. I mean after all, the object is to fill the hole, but with something befitting a nice instrument.
Post a picture and I'll pitch an idea or two.