[DonC]

Simon:
For what it's worth I think you're going to have trouble getting any kind of high torque on those nuts machined the way they are. You've only got the smallest of contact surfaces for the allen wrench. The smaller the contact area the less stress it will take before the material around the contact point fails.
Were the opening full hex formed, as in your illustration above, you could probably get any reasonable about of torque on it you'd like.
If it was me I'd be looking for another supplier of the adapter.
DonC