A spool roller/idler which is easily ("OTED") adjusted to spool widths from about 50mm to 125mm. In the blender file attached, just modify the frame for a different range to your liking or needs.
I never found a spool idler fitting my 2.3 kg spools from ReprapWorld, and I was never certain that I would get those spools again. So, I mocked up this design where the 608-guides in one side are confined while the 608-guides on the other side may slide freely on the axles in a wide range.
The frame is not too nice, and warps easily for me. I will probably make a new design which should print faster, warp less (rounded corners), and use less material. However, for now this one works very fine.
(Note: OTED, One Touch Exchange of Die, came out of development towards just-in-time production control at Toyota by a really smart guy, Shigeo Shingo. If interested in flexible manufacturing go for his book "A Study of the Toyota Production System", ISBN 0-915299-17-8. Or just get some interesting information at en.wikipedia.org/wiki/OTED)