I'm in the "if you want P90's, you want real P90's" survey group. Anything else doesn't have the voice.
Believe it or not, Epiphone P90's are fabulous. Try some out at a local guitar store. Play rolled back to 8, then full out and with a little gain. Fabulous. Very inexpensive used and very nice sound if you plan to do a conversion of a guitar you own.
If you really have 50 guitars and none with P90's, you should part with something else. There has to be some redundancy in there you don't need. Otherwise, the options are above.
That is the best tone to fun to dollars ratio out there, really. Sell that Tele you want to convert and buy the real thing. But be warned ... you'll then want more guitars with P90 magic.
Routing on a finished guitar is a bit scary.
I can only imagine, luckilyAhhh nothing to it. Heck, I once drilled right through the body of an '81 Telecaster trying to create a pocket for a humbucker mounting screw -- it was no trouble at all. Played the guitar for years like that until it was stolen.
Now, drilling a hole on a boat hull below the waterline, that gives me the sweaty palms.