Our idler wheel kept cracking so we made one out of aluminum. Works great!
As the Makerbot source says --
Outer diameter = 41 mm = 1.614in
Inner diameter = 21.9 mm = 0.862in
Thickness = 1/8 in
Start with a plate of 1/8 inch aluminum.
We wanted to make our aluminum wheel so that it would bond to the bearing without relying on glue, so we milled an inner hole that was smaller than the the specified acrylic hole, 0.806 inches in diameter. We milled the outside diameter according to the spec of 1.61 inches in diameter.
We then heated the wheel over a gas flame to expand the hole so that the bearing fit easily inside. We set the wheel on the makerbot washers to get the right height and then placed the bearing in the hole. We cooled the wheel quickly with water so that the bearing lubricant would not heat up too much as the wheel cooled. The cooled wheel has a permanent very tight bond to the bearing.