The mirror was first built in aluminium sheet, but this tarnished to matt grey, so it was resurfaces in Mylar.
The sunbox above has an internal mirror on the sloping floor that is coated in Mylar, and reflects high angled sunshine up onto the black thermal collectors. The upper mirrors, top of the picture, are intended to boost winter solar capture and do nothing at high sun angles.
The sunbox also provides solar shading in summer to the bedroom south facing windows, without affecting the view or the sunlight entry in the winter. I still have an idea to put a reflector at sill level of these, but might make it of reflective clear plastic, so that I can still look down onto the rockery.