wow, i ask for lotsa wiring diagrams.... o well.. read title ;)
Google it. If memory serves me right, there were a couple of guys who constructed their own keyboard adapter, complete with logic programming and all.
Or, you could pay $20 for a GC keyboard, use the actual keyboard for skeet, and use the little adapter that's included to use your own keyboard on the Gamecube. Quite clever of them, though a few keys are switched. I think if you get PSO, you can send for a free adapter that probably works better. Hope this helps ya!
hm.. been searching for bout 15-20 mins now and cant find anything.. u have any search words?
Well, I don't know where they went, but what they did probably took years of experience and a lot of money. You'd be better served getting an adapter, which I would hope aren't that hard to find. I got mine with a Datel keyboard, and it works pretty well.
In the end, the stress and investment of building your own would outweigh ponying the $20 for an adapter. :)