I feel your pain. I spent 3 hours this past weekend shielding my Lakland DJ5 with an Audere JZ3 preamp. Buzz drove me crazy and kept me from playing the bass at church with the in ear system.
So I shielded every cavity, then: I soldered each piece of copper tape to its neighbor with a spot solder, then I ran a ground wire from each pickup cavity to the control cavity. Then I soldered the jack ground on the JZ3 to the control cavity, and finally, I put a piece of copper foil across the bottom of each pickup and soldered a ground wire to each and then soldered them to the control cavity.
Result: absolute dead silence. Still get a minimal 60-cycle if I solo either pickup but I am okay with that. Buzz free is good!
Worth the effort IMO.
Sadowsky RV5 | Lakland DJ5 | FrankenFender 70's Jbass
Fender Am Std Dlx Strat | Christian Praise & Worship Bassist Club Member #361 | Lakland Darryl Jones Club Member #32 | Eden WTDI Pre Amp Club #26