I had an old pickup that I wasn't using, and I planned on top-mounting it to the wood. Not really all that hard. To prep the wires I wrapped the part coming out of the pickup in heat-shrink tubing so that it was really strong, as this tends to be a really vulnerable spot on most pickups. No good having a dead one. To mount it, as to can see in the first picture I drilled a small hole with a brace and bit to feed the wire through. It's small enough diameter that the pickup will cover the hole.
To mount the jack, I flipped it over on it's side and drilled four larger holes with the brace and bit again, and used a hand chisel to clean up a bit. My plan wasn't to make a museum piece, rather to finish this before the kids got bored with the project and wandered off to play football. Cavity is large enough, we're finished here.
Ok, this old BIll Lawrence pickup is perfect for this- there are no pole-pieces to worry about, so no concerns about how the string passes over the pickup. And I simply enlarged the holes and screwed it down on to the wood, passing the wires through the hole in the cavity.
And, then finally, scavenged an odd bit of plastic and mounted the jack to the plate. Here's what it looked like when it was finished.
