To add a D-ring with a rivet you'll need to drill a hole in the bottom of a G R A F L E X flash first ... tip use a wooden dowel rod inside with the same diameter to avoid bending through the bottom plate ... then use a rivetgun to attach your D-ring to the flashgun ... sometimes it's best not to pull the rivet with the rivetgun in one go but twice to secure the D-ring even better ... and when you hear a rattling sound inside a drop of superglue through the rivet hole will make it stop
Have you considered making your own? They're dead easy. I made them out of 1mm sheet steel. Never bothered with screen accuracy - but happy enough with the result.