I saw this trick on a '50's ES-175 and had to try it on my '80's LP Junior. I couldn't stand the hum anymore,
and the was not happy with the noise-free P-90s I tried.
You just cut a hole in a P-90 cover with a jigsaw and file to fit. The rout in the body is only .060" too narrow
for the pickup body. I could get it to fit just by removing the finish with the Dremel. I got lucky with this bridge
pickup because the strings were high enough that I didn't have to sink the pickup into the body.