I bought my shifter used.. with no clamp, sounds like a project for the 3D printer. The clamp turned out pretty tough, seems to withstand the abuse. Only time will tell.
Brief Instructions:
-To reinforce and add to the longevity, i left a hole in the center of the screw knob, that will accept an #8, 3 inch long wood screw. Chose something standard, you can find just about anywhere. You will need to screw it in by hand, all the way to the bottom until you feel it get tight, then give a quarter turn. Screw it in tight, but careful not to go overboard -Then thread the screw into the clamp, and once the ball of the screw is through, you can push on the pivot feature. -You will also need x4 M6x15mm hardware, i used socket head cap screws. -For better grip, I'd add a couple layers of electrical or duct tape on the face of the clamp that makes contact with the table top. -When mounting the clamp, you'll notice the screw slightly angles towards the shifter; tighten until the screw straightens out, perpendicular to the bottom surface. No need to overtighten.
Would love to see some makes, post up some pictures and any feedback would be appreciated!
If you're happy with the design, and had a successful print, AND you're feeling generous, you can tip me $1 on my paypal: http://paypal.me/bombastic3D
Appreciate those that do! Thanks