I should have done this before the Winter, as there is also a chance to stuff some insulation around the pipes as a tiny contribution to reducing system losses. In Summer, there would be heat gains through an uninsulated pipe, but I am also considering ultraviolet degradation.
This is knocked up using 1.6mm aluminium alloy, same as I used for the mirrors. Many thanks again to Mr Makita, and the maker of my Angle grinder and pop riveter. Will try get it up there at the weekend.
The cover is for this tee junction, on the right, the photo was taken when the polycarbonate still had its protective plastic skin.