I've used this material for about ten years now, on a 4 foot by 4 foot glass skylight I have here in Arizona, and it has proven to be really effective:
http://www.betterearthproducts.com/thermal-barrier-fabric-75-reflective.html- relatively inexpensive
- blocks more of the sun's rays than another material I tried.
- lets in enough light so the room still feels excellent to use.
Basically, that particular room feels good (good natural light) but would become the worst room in the house during the summer if not for some effective material, and this material has seemed pretty good. [I won't say it's "the best" since I haven't made a thorough survey of the space, but I'll just say as far as I know it has seemed to serve my own needs quite well.]
Live-and-learn:
I recently belatedly realized that I have another couple of smaller tube type skylights (one a converted chimney). Their openings are much smaller, but I will apply the material to them (for next summer) and see how it goes then, to help reduce my cooling bill. I also turned some over to someone building a greenhouse recently, so I'll see what they say about how that worked, for the harsher days when the sun is just too much.
Note - I am not associated with the company in any way, I just figured I have found the product to be a relatively inexpensive "keeper" and so for those that live in hot climates, they want to know about it, or something similar.