The problem lies with the design of the Duss drilling stand. I have the same issue, you cant compress the ring enough, it only compresses 0,2mm
and most other drilling machines do not have good enough tolerances to fit in the Duss stand.
Especially the design of the clamping mehcanism is bad since it has a bridge in the open slit (dont know why it is there) and it limits the compression.
I just asked a metal workshop to build me an alternative clamp so I get around the problem. You could also try cut through the bridge between the holes
maybe that will alllow enough compression but again I am not sure if it is a bad or good idea. I tried adjust my drill neck diameter with tape but that
is too soft and you can still turn the machine in the drill stand.
The last alternative is to buy a Duss drilling mahine for 400 Euro which is way too expensive for me and limits you to using only Duss machines.
I am hoping with my new clamp I will be able to use any machine with a standars 43mm neck regardless of tolerances.