Anything with an internal diameter greater than 1 1/8" (1.125") would work.
Schedule 40 PVC, 1 1/4" nominal has an OD of 1.6" and an ID of 1.138". Should be just the ticket.
Your challenge is getting a nice straight cut.
Another option is to use a second stem. I do this as a permanent solution for my touring bike.
And still another option is the Sheldon Brown kludge
Scroll down to "Threadless Without Spacers"
This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.
Last edited by dobber; 06-19-08 at 03:15 PM.