This is probably already too late for some folks, but I came up with my own eclipse projector, and tested it yesterday. And it seemed to work pretty well. It works on the same principle as the cereal/shoebox pinhole projectors, but allows others to enjoy the show too. This will require some adult supervision (and a little common sense) to set up.
There are two parts to the eclipse projector: the screen, and the projector.
To build the screen, start with an ordinary banker box (without the lid). Spray paint the entire inside of the box matte black. Then tape or glue a regular piece of white letter or legal-sized copy paper into the bottom
of the box. Put the box on a TV tray or card table, preferably in a shady area where people can watch the show without having to stand in direct sunlight. Turn the box on its side so the white paper in the bottom faces out toward a sunlight area. This is your eclipse screen.
To build the projector, you need a small ordinary 1x magnifying mirror, preferably one that has some kind of base or hinge that will allow it to be more easily aligned to your eclipse screen.
If you have steady hands, and don't mind standing in the sun though, then any small 1x mirror will do.
First, make sure the surface of the mirror is completely clean and dust-free. Then cut or punch a small hole no larger than 1/4", and no smaller than 1/8" into a small piece of black cray paper. And tape that paper to the surface of the mirror. Then make sure the rest of the mirror is completely covered with black cray paper, except for the small hole
. Avoid glossy transparent tape because that will also reflect some light. The basic idea here is to create just a small "pinhole" sized mirror that's no larger than 1/4" in size.
Put the mirror on another TV tray or card table in the direct sunlight a good distance away from your eclipse screen. And angle the mirror so that it reflects the small pinhole of sunlight onto the white paper screen inside your banker box. Make sure noone is standing in front of the mirror when you do this, because a direct reflection of the sun can also harm someone's eyes.
Aiming the mirror is probably the most difficult part of the setup. You can make it a little easier though by initially holding a white card in front of the mirror to better see where the pinhole of sunlight is being cast.
Also remember that the sun moves, and you'll actually be able to see this movement in the reflected image on your eclipse screen. And you'll need to make small periodic adjustments to either the position of the eclipse screen or the mirror to keep the image more or less centered on the screen.
The smaller the "pinhole" is on the mirror, the sharper and also the dimmer the image will be. And the farther the mirror is from the screen, the bigger, and dimmer, and more detailed the image will be. So you may want to experiment a little to find the best configuration of the two.
You could also try a magnifying mirror (5x magnification is plenty), to make a somewhat bigger image. But it may distort the shape of the sun, and make it less sharp and bright. A regular 1x mirror seemed to work best in my tests.
Be safe, have fun, and enjoy the eclipse folks.