Adapter for a M5 screw for a 6mm hole
One of my racks has similar oversize holes and just doing the bolt up tight is fine, with a washer under the head. Some clearance in fact is necessary (the typical clearance hole for M5 is 5.5 mm). I've also seen plastic spacers in this role. If you find it needs just a little something to fill most of the gap, heatshrink sleeving might do the trick, again with a washer under the screw head, as the spacer is centring rather than bearing a load. It is possible to find aluminium tube of just the right size whic could easily be cut with an adjustable pipe cutter used gently, or a hacksaw (I assume you don't have access to a lathe or this would be trivial).