Since I'm soon to go to 33's on a 35 I've got a plan for something similar. It's not an original idea, but I forget where I stole it from.
2 steel tubes - close to the diameter of barbells - make sure the outer diameter of the smaller is a close tolerance to the inner diameter of the other.
Here's the idea - cut the larger tube into sections. Use barbell stops (the kind with a set screw) to clamp the sections onto the inner tube. That way you can adjust the "rollers" to fit different diameter tires, or even move them forward/back depending on where it rides naturally on the tire. Should only cost about $40 bucks for the steel and parts. My thought was to make it adjust from a 31-35" tire.
