I found a website in Brazil that gives instructions for building your own sextant (for navigational practice only) using a CD and Lego blocks.

http://www.tecepe.com.br/nav/CDSextantProject.htm

He points out that if you're going to do sun-sightings, you could easily lose an eye; he uses two pieces of "dark negative 35mm film material" mounted in a 35mm slide holder. I'm not going to trust my eyesight to such an untried and untested solution. What could I use instead?

I thought maybe those sun filters that science museums sometimes sell for viewing solar eclipses, but where do the museums get them from? And would they be adequate for use in a sextant?