Thanks, yes, that's another option, especially if plugged internally and also reinforced externally with an alloy jig, which I think would need to be permanent.  Otherwise I think I would need a couple of 45 degree mitres to avoid a sharp corner and that these would need both internal and external reinforcement.  I thought an ABS fitting could act as a former, with fewer point loadings than a mitre, and provide the inner layer of a sandwich, but I was wondering if anyone had tried that.

Ok. .  It's a 'hoop' for a Merlin Rocket sailing dinghy.  Two uprights approx 900mm, cross bar at top approx 400mm supporting a short section of mainsheet track and traveller.  Must be able to withstand shock loadings (and me hanging on to it when I lose control of the boat!) of approx 70kg. Hope that makes sense.  Here is a picture of one without track: 
https://goo.gl/images/i59tCn