Anodized 6061 alumimum hex spacers, measuring 5.5mm from edge to edge (not to be confused with the length, which is also 5.5mm). M3 inside threaded on both ends. Beauty doesn't come cheap.
Please note that screw length is critical with a 5.5mm length spacer. You want at least 2mm to get three turns of a screw into the spacer at each end.
Package of 8
0.28g per spacer