Just noticed this thread.
Thought at first you were going to coat the surface of the dish with aluminum foil or surplus mirror pieces and build a concentrator, then saw you using it just for one-axis tracking.
If you decide to try a concentrator and 2 axis tracking, the link below is a unit I helped build that could give you some information of the thermal aspects of keeping a concentrator cool.
http://www.stretchedlensarray.com/Papers/SPS600V.pdf