A common issue when shooting into bright sunlight or rim lights is glare. Of course many of us use lens shades and/or lens hoods, but Strobist finds a simple frame can control glare even better. He illustrates what he uses, and the materials used, but he also warns us to be careful what size you use if you want all around protection. Your aspect ratio needs to be correct, but experimenting is fairly simple.
Strobist writes:
Whether you are shooting into the sun or working with rim lights it is a good idea to flag the backlight to control glare. But rather than putting a big flag on a stand (on each side) near the subject, you can get even better control shooting through a hole in an opaque material.