I use 6mm multiplait. The shackle needs to be the narrowest you can find with no protruding pin, otherwise it snags on the inside of the rudder I found those bent plate ones used on dinghy rigs are the most suitable. I notice that my rudder is not quite the same as Aztec's, though. My blade doesn't have the cut out shown, and it also has a short 'tail' at the after end to act as a stop. There is also a small sheave inside the inspection hole that gives the line a better angle for the final inch or two of lift.
Neil Sinclair
Seal 28/27
'Andiamo of Exe'