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.