So I got round to fitting the polycarbonate windows yesterday. I wanted to utilise the existing clips that held up the glass windows and fabricate my own bracket to hold up the new plastic windows. Using 3" bolts and some washers I was able to fabricate a bracket which attaches to the inner door skin and can be adjusted to push the window up or down and in or out so I could get a tight fit on the standard window seals.
I'll take some pics of the brackets once I've completed the fitting, the top part of the window is still loose as the window kit I purchased was flat and not formed so I may have to using window seal to stop the top of the window flapping around, or I may pop rivet it in place around the top frame for total security (so long as it doesn't crack).
Here are some preliminary photos of the drivers side window fitted.