Backstory - I swapped out my 6800 short cage for a long cage 6800 to get a bigger cassette in. Then shimano released the r8000 long cage, and it allows an even bigger cassette.

I don't want to buy a $75 r8000 long cage if I can help it. What I was interested in, is all the people selling used r8000 short cages for cheap (as more people are going to long cages than to short), if I can fit one of my spare long cages off a mtb.

I have

1) OneUp long cage
2) XT 10 speed long cage

Pretty random request, but does anyone know if this will fit? If this could work, I could come close to breaking even selling my 6800 long cage and buying a r8000 short cage to convert.