I needed a tiller extension for an Aqua Finn (Sunfish clone) but I didn't want to spend too much $$.
I ended up with a 30" section of 7/8" outsider diameter PVC pipe and the hammered in a 5/8" hardwood dowel to give it a little more stiffness. Then I cemented on two end caps, and attached it to the tiller handle a stainless steel machine screw. Total cost: under $5.00. It has a nice feel to it and should last for a good long time.
Great idea. I have been using a golf club for my extension but it is a very small diameter and I feel like I don't have good control in high wind. I think your idea could really help that, but how would I attach to the universal joint? The stem on the universal is smaller than 7/8". I guess I would drill a hole in the end of the wooden dowel........
If we are talkin inexpensive, here is my version on the old style tiller. I used a modified leaf rake handle. The plastic acorn nut keeps one from scratching the deckpaint.