i have a cross bar that is 1.6 mm wall alloy 1" tube what gary and mitch use, i have put a plastic flush in the end of the tube maybe 3 to 5 mm hanging out and drill in the center of the plastic and put in a Riley swivel and pop rivet it in, to line up rudders ajust the plastic or the swivel till you have measurements right, i found its better to use the riley fittings cos they flex, if you happen to lean on them. plastic cant handle the weight.

Matt