Years ago there was a guy here making them. But I can't find the thread(s). I also recall John Walker sharing his solution to extend end of the 915 shifter lever. I copied it (as he predicted people would) and I have to say it's quite a noticeable difference in a good way
http://forums.pelicanparts.com/porsche-911-technical-forum/240038-amazing-long-shifter.html
http://forums.pelicanparts.com/porsche-911-technical-forum/727654-moving-shifter-back.html
What I noticed most is that my habitual memory takes a long time to change. I still find myself reaching lower for the shift knob. But that's a small personal problem for me I suspect. The positive is that it's pretty cool to have the shift knob positioned so close to the steering wheel. When you take your hand off the wheel to shift, the knob is right there.
JimmyT here on the forum makes a really nice extension piece with an ingenious method of installing it via a long screw using a long expansion nut that cinches it into the original shifter tube. Everything Jimmy makes is top shelf quality & design. Reasonably priced too.
http://forums.pelicanparts.com/porsche-911-technical-forum/820219-anyone-make-shifter-extension.html