We are in North Texas and the West sun blazes against our windows! What we did this year, because the Solar Screens helped but with that kind of sun you need a LOT more, we got the bubble wrap that is reflective on each side and bought the tape so we could cut and tape it to fit our windows. It is insulation and found by the insulation at Home Depot.
Then we measured each window so that we could slip the reflective insulation between the solar screens and the windows and so they would be reasonably easy to remove come October, when the heat isn't an issue. So make it an inch smaller than each side of the window. You will get a little solar gain this way but not much, otherwise you will never get the stuff between the screen and the glass.
It is like foil in that it reflects but putting it between a solar screen and the window you can even see the shine.
It took about and hour and half cutting, taping and installing it. We had 6 good size windows and one small one in the upper bathroom. And, although you can't see out those west windows, it has made a HUGE difference in the heat upstairs.
On the lower floor I am making awnings lined with this stuff, so it looks good on the outside but under the fabric it is the reflective stuff.
I think we have about $80 invested in it and I had enough to make an bodacious solar oven!
I wouldn't use anything clear because it is the radiating heat that gets magnified through clear glass or plastic, think of your car in the summer and how it heats up through the glass. That is the same way clear plastic would work. ANYthing Reflective needs to go on the OUTside of the windows if you have double or triple pane windows because the reflection will pop the seal on those windows.
Here's a photo of my solar oven before I added to flexible wings to it, you can see what kind of material Im talking about.