Projecting an image of the sun / eclipse This might be an astronomy.stackexchange.com question, but I think it is more in line with a camera obscura.

During the last eclipse I didn't plan ahead and grabbed some lousy toy "spy" telescope we had (about 2 inches long), set it up on a tripod and pointed it at a white foam board.

I'd like to plan ahead more this time.  

What can I use to project a decent image of the sun onto a foam board?  What kind of lens do I need?  Would a magnifying lens work?  How about a close-up diopter filter?  How about a 50mm lens?  (I'd like to use something I already have, or is cheap -- and I don't want to use a pin hole.)

My first thought was that a magnifying glass only focuses the sun down to a pin point (and would burn the board), but I'm recalling now that past that focal point you can project an image.
I've done this using two lenses.  In my case they were a 300 mm and a 50 mm.  I built a jig to hold the lenses in-line with each other, to allow for small adjustments in the placement, and for a tripod mount.

Put the long lens, in my case the 300 mm, in front normally.  That will form a focused image behind it.  Put the short lens backwards behind that.  This lens is used in reverse.  It takes the small image created by the long lens and projects it out what is normally its front.

The ratio of the front lens focal length to the back lens focal length is how much angular "gain" you get from this setup.  In my case, the cone of project of the sun was 6 times wider coming out of the 50 mm than going into the 300 mm.  This means you don't have to get as far away from the overall setup to get a decent size picture.

I'm not where this jig is now.  I'll try to remember to take a picture of it and post it here within the next day or so.

One thing you'll find is that the earth rotates annoyingly fast when you are looking at a magnified picture of the sun.  Use a telescope mount if you can.

Another issue is blocking out as much ambient light as you can.  The projected image of the sun is reasonably bright, but so is daylight.  Some sort of baffle over the top and the front except for a hole for the front lens helps a lot.  People can then view the projected image from the sides.