Lindy Fralin builds two 5-string pickups for me under a P90 cover with no holes. The first is his Big Single, a sidewinder design that uses a magnet in top of a mild steel blade. The magnet used in the pickup is a bit of a trade secret and it's not what is shown in the photo.
In order to accommodate a 5-string bass, I had to have wider blades cut from mild steel. I also need to cut and glue together guitar bobbins. I made up a cutting jig and use a blade wrapped in packing tape as a gluing jig. These are ready for winding and Lindy's shop will take it from here.