Still having trouble with buzz out of a P-90 pickup - is there a different type of PU out there having less buzz issues, with the same physical dimensions (TV Jones?, etc) that would drop into the routed space for the P-90?
FYI, many humbuckers can be made to fit a P-90 route. In most cases the coils will fit just fine, but the base plates are too wide (not all, Epi, and GFS have coils that are too wide to fit). Trimming the side walls of the base plates enables most 'buckers to fit into a P-90 cavity with little to no alteration whatsoever to the guitar.
The pictures below were taken to demonstrate that this was possible to do. The guitar is a 1977 Les Paul Special, and is bone stock.
Obviously, if you were to mount the pickup as I have shown, you would have 4 screw holes from the pickup ring unless you mounted the pickup to the floor of the cavity, ala EVH.
Trimmed pickup (a Gibson 490T)
Pickup dropped into the cavity, and mounted to a ring...
Do you want something with the P90 tone or just something the same size?
I have had great results, installing a TV Jones Classic (Filter'Tron clone) in a PRS SE One. TVJ offers 'Trons in P90 covers. (Twelve pole screws showing.)