Been in NZ for 3 weeks and had generally limited internet.
Anyway, How good a curved surface do you need.
It depends. Its not high quality optics level, but if you need a high concentration ratio they need to be pretty good. Also there is usually a non-imaging optics 2nd stage focuser.
non imaging optics are very cool. again if after a Google your interested i have some papers on them.
But for low level focusing the only problem with a poor reflector is uneven heating of the surface. this is not really a problem for low temperature absorbers (ie copper <1000 C).
I have used a hand made Al foil and got to 250 C will a little insulation on the absorber. It was pretty bad so doing better should be easy.