Hi all,
I've cut some out of a stainless steel sheet with a guillotine, they tend to mark with water splashes etc. but can be bent into a parabolic shape. I filed and taped all the edges, so it is a bit more work than buying them.
You want stainless steel grade 304, 0.5 or 0.9mm thick, mirror finish. I had an off-cut from a radiant heater reflector, but you can buy stainless steel plate in small amounts of ebay etc.
I also tried aluminium, but it would keep a bright finish.
My newer ones use are "shed guttering" painted with gloss white paint, and the tubes tied on with cable ties. These work very well, and are very easy to make. I'm a cheapskate and used whatever paint was to hand and ended up with a horrible streaky effect full of runs, but if you used car spray paint and clear lacquer you could end up with a more professional looking job. They also come with ready made ends.
Apparently "gloss white" offers as good a reflectivity as polished stainless steel, the ones I had were white "Freeflow Full Gloss Rainwater", so I probably could have got away with not painting them at all.
cheers Darrel