When you say "telescopic" do you just want something that extends, or extends and locks?
I use the standard 1"x1" aluminum tiller housing and build a wood box around it as part of the tiller body.
There's a not-great description of how I make tillers at:
http://geoff-s.net/ice/FoamCoreTiller/For the locking system, many people build a mechanism that allows the end of the tiller-housing to rotate (usually a ~6" long section).
You pull the tiller out and twist it 45 degrees to lock it in the long position for the start, then turn it back and slide the tiller forward as you climb into the boat.
Matt Struble had a very simple catch system that that locked the tiller in the extended position and then unlocked using gravity when pressure on the tiller was released. I can't find a photo of it anywhere...
I'm going to try a really simple pin system on the tiller I'm building for this season. So far it seems to work very nicely, but I haven't tried it sailing yet so I'm not 100% sure there won't be a problem with it.
Here's a short video showing how it works with an unfinished tiller:
http://youtu.be/yfs-ksjoudQCheers,
Geoff S.
US-5156