OK, the technique is pretty simple, I'll leave most of it for you to ponder... but the concept is simple:
You can "trace" any pattern (or actual unit to be installed

) with a flush trim bit - bearing rides the pattern, cutter cuts out an exact duplicate... but the duplicate shape is a positive, and you need a negative.
But what else did you cut out?
...an oversized negative, didn't you?
Flush trim bits come in different diameters - but all of them standard... like 1/2", 1/4", etc.
Your oversized cutout is exactly 1/2" oversized, if you used a 1/4" bit.
Look at that master bearing kit... you can remove the stock bearing on a flush trim bit, and replace it with an oversized one - and create another negative that is shrunk down - with a 1/4" oversized bearing, you'll recreate your exact size hole, a negative of your positive original pattern.
And, if you use one of these kits with a rabbet bit instead (or work off a positive, the inverse of this trick), you can do enlargments rather than reductions.
Anyway - it's so accurate, that if you are going to upholster using vinyl - I suggest (in fact, require!) that you wrap your original positive pattern with the vinyl you are going to use - just wrap a stripe of it around, so the bearing rides it on it's original pass.
Now, when you reduce it, it'll be the size you need, minus your vinyl - and when you add the vinyl, you'll end up with such a tight pressure fit, you could mount it in your roof and not have it drop out.
