My advice to you is build your cafe seat on the existing pan that you have. It'll obviously fit your frame with no mods whatsoever. You can just cut off as much of the seat foam as you want, and make a cowl for the rear. If you can't hammer one out yourself from 14 guage there are a few options:
You can cut down a rear fender from something like a Sporster. You can cut the rear cowl piece from a Dime City Cycles seat pan. They're around $79 on eBay. You can take it to an upholsterer and have them add foam to it in the shape you want and make the cowl from leather. This looks better than it sounds. Burly Brand make a Sportster cafe tail like that.
Lastly, you could try your hand at fiberglassing and make one. I
think there are many examples right here on this site showing how that's been done.
If you do any of the above you'll still be able to return your bike to stock condition any time you like. (like when taking the CFO out for a spin) To that end you'll need to keep an original seat. Oh, what a co-inky dink- I have a '79 CB750 K seat in very nice condition...
I put the "G" in Jihad.