When I put the OBP-2 pre in my P-bass, I just used a wood chisel to remove enough wood for the battery.

Obviously, if yours is a true older instrument, you would not want to do any modifications that might harm the value. Mine is a 'beater' that I've modified a couple times. FWIW I've since replaced the PU with a Nordstand and removed the preamp. I found the OBP-2 to be (as many on this forum have said) lacking in the mids, so I sold it and moved on.
Good luck!
