Aluminium Batten Fencing

Hi there, I’m in Australian, I am currently trying to specify an aluminium batten fence with 20x20x3mm verticals and 80x50x3mm at 80mm centres and posts at 2400mm intervals. What I don’t have is a rail dimension, I have tenderers advising their fence fabricators don’t want to be liable for specifying them. I would have thought they would know their products enough to make this call. I’m not sure what to specify so I thought I’d post here to get an educated opinion. I will also be contacting colleges I used to work with.
Thanks
Josephine