Re: Mini-humbucker in Soapbar p90 space
The way Gibson did it on LP Deluxes (which used mini hums in a soap-bar rout), was to make the pickup and a P90 cover into one unit, which was adjustable up or down via mounting to a tapped metal bar, or to threaded inserts sunk into the body.
This is achieved with spacers, nuts, and solder. Look up online how to do it. You can fairly easily fabricate your own mounting bar with small piece of metal, a grinder, a drill (press, preferably), and a tap.
You can also add hold-down screws to the corners of a P90 cover, and shims as necessary below to mountain it it at the right height, then just mount the pickup like a full sized humbucker, hanging from the ring.
Or, you can buy the GFS conversion rings. They're cheesy, and require screwing straight into the wood, but they'll do in a pinch.