However, for whatever reason, that's not the case for me . The mounting arms simply won't fit between the metal sheet of the rack and the support tube, so unless my bike had an extra 4" of chainstay, I can't get it to work. Does anyone have a good way to fix this?
I'm slow, are you saying the supports extend further forward and they don't fit under the support bar? That's ok, it's the metal that's attached to the rack that matters, the part extending beyond the screws has no purpose since the rack is supported vertically at the eyelets.
1995 Kestrel 4000; 2013 True North Touring; 1989 Miele Tivoli; 1979 Colnago Sport
I just looked at my identical rack. There's only barely enough room for the strap (it scrapes on both sides when you slide it through). You can hammer the surface of the rack to make room, cut the excess strap off (not recommended in case of future use on another bike), or bend the strap below the tube.
I recommend the hammer.
Edit: I see you've already fixed the problem with the hammer method. Good job.