MIMF

I'm getting ready to "dress" the J-bass I've been making for what seems like decades, but is only years. The guy I'm building it for gave me a bunch of random hardware that he had collected, including some individual bridge/saddle units. You mount them beside each other but they don't touch. I'm having a hard time coming up with a graceful way of grounding them (4 units) that doesn't include running a wire on the top of the instrument, or drilling four long holes into the pickup route, in the certain knowledge that I will screw up at least one of the holes. Has anyone ever seen these things? They look like these: https://www.amazon.ca/FLEOR-Individual- ... B07QKYV4M1

I think each of the 4 bridges needs to be grounded but I can only think of two ways. 1) Somehow run a wire to a mounting screw/bolt on each of the 4 units. 2) Make a base plate out of a conductive metal to go under the bridges.

I thought of one more but it may be a bit hair-brained. Drill a small hole angled from the top of one outside bridge mounting hole in the guitar body to the bottom of the other outside hole, crossing the two interior holes. Stuff a piece of solder into the angled hole then install the bridges. Check continuity between the mounting screws.

Any of the above would require connecting to the ground of the guitar at some point in the chain. The normal place is the control cavity soldered to the back of a pot. Sorry if any of this is obvious.

How about using through-bolts instead of screws, with a single drilled and tapped backing/grounding plate hidden in a cavity from the rear?

This suggestion probably too late for you as you've already made the bass. I've used those on several basses. It's another reason I like a zero fret (besides making nut fabrication much easier) - ground one of them, and as long as you're using metal strings, the others will be grounded through the fret.

Ground one, and mount the bridges on top of a strip of copper shielding.
Or use a brass nut.
Or a metal string retainer behind the nut.