on gen3, one left side is 28mm (too small for most 1"1/8 = 29mm accessories out there),
right side is 34mm (usually too large).
For the left, I hacked a 1" (25.4mm) pvc pipe fitting. I did a neat job but I wouldn't recommend this crafting to avg joe. I would recommend a delicate dremmel sanding with the 1/2" drum they usually have, and go around enlarging the hole imperceptibly until it fits 29mm.
For the right, I cut 2 plastic washers (about 1/16" thick) from a garbage bin (leftt over from my DIY front fender extender)
About 2" diameter to start with a 20mm hole to enlarge to requires specs, again with dremmel delicate work. If the side freeplay (in the 34mm hole) is not handled with properly tight assembly, you can always put rim of double sided mounting tape behind your back washer (if you ever sell and the mounting tape doesn't comm off, at least it's inside thus invisible).