Neewer Matte Box

Check pricing and availability: Amazon

Dirt cheap at $20 from Amazon, you're going to have to fashion your own donut and -- depending on the situation -- plug all that daylight between the box and the flags.  For less than $1.00, a hair thingy called a scrunchie is an easy alternative to a donut. If your budget is north of cardboard and duct tape but south of its more expensive but still cheap bigger brother for $90, this unit is a good place to start your exploration. With this written, you'll surely graduate to something more robust and featured once you discover the value of a matte box beyond what a simple lens hood can give you!