With a bit of strolling around on this forum,you're definitly gonna find enough info on this subject.
But to give you a start,this is what I did.
I mounted the gripshift (Shimano 3-speed) on a piece of solid 22,2 mm rod,which was threaded on both ends. That way it was mounted on the frame (you can use a clamp for that purpose).
Then I used a clamp with a welded-on nut (which is for the shifter-stick) and put it right around the rubber gripshift.
I admit that it it not the king of beauty,but it's working great for the past two years now.
If you want more info,just give shout. Maybe I could make some pics.
Good luck on your quest. :mrgreen: