I can buy a inner and outer long cage plate, so a GS/medium cage that's a good price would work.
Otherwise, I'm considering fitting the XT long cage to my SLX M7000-11-GS. I haven't been able to compare that particular XT rear mech to the SLX to see if it's just the cage that gives the XT M8000-11-SGS it's 47T total capacity.