If it is only for temporary use, you could use many materials. Traditionally, tillers are bonded layers of wood cut and shaped into the length and arch needed to clear the transom and knees in the cockpit.
My first choice would be to get a used tiller on craigslist. Just make sure it is close in length to the original. And make sure you can connect your hardware to it.
If I were making one for temp use, I'd use aluminum tube with the tangs welded to it for attaching to the rudder head. A wood tiller needs more maintenance than a metal tiller.
Probably need 1 1/2 - 2 inch OD tube with 1/8 inch wall thickness.
I think others can add their experience and expertise here because there are some great fabricators on this forum. I bet someone even has plans and pictures.
You could also YouTube it?
Thanks.
Andrew