A couple things just don't make sense here:
1. You want to use a genuinely shaped P-90 because you think that altering its shape is going to change its tone.
...Do you realize that there is a lot of difference in tone in various true P-90s? There is not just one sacred tone?
...Do you realize that there are many humbucker-sized P-90s that will duplicate those tones exactly of any "real" P-90?
2. You are not going to "drill any additional holes or enlarge the existing HB hole" in your L-5.
...But you're willing to mount the P-90 on top of your guitar and do a lot of work to fill-in the "gaps" of the existing humbucker route.
...You think that somehow you're going to mount a P-90 on top of your guitar and it will sound like a P-90 mounted correctly....... and it will look OK?
3. You imply that you're advanced enough to be making some humbucker-sized P-90s of your own,
...But you don't understand the size and shape differences? That makes no sense.
...And what do you hope to accomplish by this feat? You are already convinced that you can't get a "genuine" P-90 sound from that altered shape. So, why?
4. "worried about the authenticity of the sound. [I grew up playing a '54 LP Custom.]".
...You're judging all other P-90s (including humbucker-sized P-90s) by that "authentic" P-90 tone, but I guarantee that even if you took those exact same P-90s from your LP and put them in your L-5 they will sound DRAMATICALLY different.
5. You say "No point in re-inventing the wheel".
...But that's exactly what you are doing by making your own bucker-sized P-90 pups.
I hope that you also realize that in your L-5 you're probably going to need a P-90 that is wax potted (that may not have been necessary in your LP).
Look,
There is no one single "authentic" P-90 sound.
You are going to destroy your guitar trying to do what you are planning...you obviously don't have the luthier skill or experience to do such a thing.
There are MANY humbucker-sized P-90s already available that are absolutely excellent sounding (as good as any "authentic" P-90 made). idsnowdog listed several very good sources.
It's a simple task to just buy a bucker-sized P-90 and mount it right in your L-5 without altering or hurting it in any way, and get that genuine P-90 tone that you're looking for.
I am a real big P-90 fan myself, so I get your desire to put them in your guitar. I totally love the bite and aggressive tone that is part of the P-90 sound. There are some general distinguishing characteristics among nearly all P-90s but a lot of variation in tone exists as well. I love the fact that there are 2 magnets in a P-90 which allows me to customize them even more than a bucker. I love the versatility of guitars with humbucker routes, and I love the fact that there are many humbucker-sized P-90s available to choose from to get the exact P-90 tone that I'm after from nearly any guitar.
You've got lots of choices available to you to get your guitar sounding like you want (and looking good, too). Don't muck it up with your hair-brained idea of mounting a P-90 to the top of it.