Windjunkie;
I have to agree with Basil on this one. Mirrors are expensive and Mylar will give you much more controll over the curvature of the reflective material.
The one item that will make just as much of a contribution to the working product will be the focal point.
Builder beware, once you start getting even a few sheets on it; anything,person,part remotely close to the focal point or any sub-set of it will quickly find a burning sensation.
A 15 foot dish reflecting all that sun into a specific point will equal 15+ suns worth of heat/energy in seconds.
Did a small test using an old 3 foot Dish given to me by a good friend. I lined it with mylar, setup an old cast iron pot at the focal point, then turned it into the sun. I reached for my morning coffee and by the time I turned around, the old seasonings was already starting to smoke, the temprature <using an IR meat therometer> read 250F. Took me little more than 5 more seconds to turn it back away and the pot was smoking !!!
Now of course this was a good warm summer day with lots of pretty sunshine, but you get the idea.
When designing this, make sure and use high-temp everything, knowing that temps can get up way up there in mere seconds, and you might need a good "robust" pump to move the water fast enough to keep things at a low enough temp.
Might be a good idea for a hot tub :-)
VERY doable, but do be careful :-()
Cheers
Bruce S