I've done some pickup cavity enlarging totally by hand. Chisel, sanding blocks, etc. I started this project, and took pictures for "TalkBass Forums". Most people were skeptical, and were expecting a disaster.
I like to think that I proved them wrong.
Just take your time. If you're using the chisel, line up your tape with where the edge will be, but dont chisel all the way up to the edge. Take the last 10% off with sandpaper, to make sure that you don't chip anything.
Standard J Bass bridge route
Putting in a double-J bucker. Measured, and put down some tape.
Just started taking away a little bit of wood at a time, with the hand chisel. Make sure it's super sharp. Again, don't go all the way to the edge.
Just keep working at the edges with sandpaper. Wrap around a pen, or dowel if needed for corners.
The end result, even up close, looks damn near stock. I'm sure that if I TRIED to do this with a router, the thick poly would have chipped like hell.