You're better off doing it yourself as the premade ones have the humbucker too close to the bridge and this results in a brighter sound than any of kurts mustangs. Kurt's still have the pickguard screw in front of the bridge, the humbuckers are so close to the bridge on the Japanese ones this screw isn't even there.
I've compared a lot of photos of the Japanese ones and Kurts ones and the difference is big enough to effect sound so if you're after a guitar to sound like kurts,the japanese ready made ones wont do it.
I would buy just a regular mustang, a humbucker template set from stew-mac (and a router if you dont own one) and do it myself.
Its easy to route guitars and pickguards for pickups especially when you have templates.
As for the actual humbucker location, I have a big pic I made up of the exact spot kurt had his in if you need it for reference.
