Thanks PZG ..
I called a mob in QLD and they told me that i should not use Lexan, but instead acrylic, as; 1) lexan scratches more easily, and 2) acrylic is easier to work with. They said they take an factory piece of glass, lay acrylic over it and then place the whole thing in the oven to uniformly heat it. The heat allows the acrylic to take on the form of the glass, the edges trimmed and voilla! you have a new light rear screen.
The point I wanted to highlight was that acrylic was suggested as the preferred item over lexan, and i think the generic name for the acrylic item is perspex.
Watch the brown hornet blog to see what I end up doing, as at this stage i need to check prices and availability in Sydney.