I just finished up a build that called for direct mounted pickups. I wanted these to be adjustable but more importantly I also wanted to preserve the threads in the pickup baseplate tab threads that normally get screwed up or drilled out by using regular wood screws to mount the pickups just in case I ended up pulling them out for any reason.
I ended up using 2-56 Brass screw-to-expand inserts along with matching half inch 2-56 thread pan head screws from McMaster-Carr. These simply press into an eight inch pilot hole and are reinforced with a drop of CA glue.
These screws are slightly smaller than the existing 3-48 threads in the pickup tabs. They do catch the existing threads just a bit but they will screw through the holes but will want to grab the pickup threads when fully inserted. To avoid any thread damage to the pickup I filed off about an eight inch of threads just underneath the screw head so it wouldn't damage the existing pickup threads once fully inserted and turned while adjusting. I did this by chucking up the screw in a drill and using a file under the head.
I tried using these with two layers of foam under the pickup but the pickup could be pressed down pretty easily and felt kind of spongy. I fixed this by adding two pickup mounting springs cut in half. They are now quite firm and can be adjusted infinite times without worrying about the screw stripping out of the wood.
Pretty simple but it works quite well.
John is a long-term contributor to ProjectGuitar.com, a popular Guitar Of The Month winner and a good guy all round. His sublime boutique carved-top guitars include the signature Dimple model and are sold under the John Wallace Custom Guitars marque out of San Diego.