I just made a (double) tiller extension out of PVC conduit using a "flexible elbow" in the middle, clamped to the middle of the tiller crossbar. The result is similar to the arrangement in Rick's link, but each side is independent.

I used 3/4" conduit because it matches the crossbar diameter, and 1/2" seemed too flimsy. The "flexible elbow" is a single item, next to the rigid conduit elbows on the store shelf. I used two pairs of the plastic conduit hangers (side-by-side & back-to-back) for the clamp, and I put solid caps on the ends. I paid about $13.00 total for all the components, including nuts, bolts and star washers at Lowe's.

I made each side 4 feet long (sticks out 1 foot beyond the tiller arm) and put a ball-bungee on each side, to hold the un-used half snug to the crossbar (out of the way). The "flexible elbow" includes threaded couplings, so you can unscrew the poles for storage (or IWCA regattas, where extensions are prohibited).

With the short EZ-Lock tillers, I really felt restricted to the back corner of the boat. Now I can move up or hike out as much as I want.