If you're considering the DIY route, this is the stuff I used. It's just a reinforced Mylar. Ignore the posted weights; they lie! Actually, they just use a different measurement than we are accustomed to using. If I remember right, the fabric weight was roughly double the .75 listed for a square yard.
No sewing required or recommended; just use the sail tape. I've had mine for a couple of years and am always amused by the amount of use it gets. Seems to be holding up quite nicely. It is VERY stiff and compresses not at all. You certainly can't see stars through it as it tends to be sort of 'milky', but you can see movement and light sources without problem.
Here is my girl standing under mine: