The ropeman is the way to go for a rope linesmans belt.
I went with a 9mm Bluewater rope and tied
It like a spare tether.
I'm pretty sure the ropeman1 is only spec'ed for 10-13 mm ropes...
It is but there are several people here that are using it with thinner ropes on their tether.
It holds my Bluewater 9mm without slipping or crushing it. I have only used it as a tether for brief testing. But as a LB I am confident with it.