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?

You are right about not trusting the 35 mm film. When we had an eclipse over here, officials issued a specific warning that neither that nor glass with soot should be used as filter. The filters for watching an eclipse would be adequate.
I found a set for building a practice sextant at amazon. Here is a link: sextant
The set is made of cardboard and requires assembly. I own one of these. It´s pretty easy to build and seems to be accurate enough for practice. If you go for it, you may wish to get the bubble horizon too.

I use a pair of welder's goggles for my periodic pagan sun observations. If I'm going to join the unwashed masses staring at the sky there better be food , potables and pretty girls <img src="/images/graemlins/tongue.gif" alt="" />. There are several relatively inexpensive plastic sextants for basic instruction. Just remember when shooting the horizon to stand very straight with one leg bent on a step,log etc. This won't help steady your observation, but you'll look like a dashing younger Farley Mowat <img src="/images/graemlins/cool.gif" alt="" />

Here's a good example why you shouldn't use film etc. Several years ago there was an eclipse and many employes came into the x-ray Dept. for old exposed film to fold and use for viewing. Later that day there were a lot of people with minor eye injuries. Even one of the E.R Docs. had some eye pain. Check with a place that sells telescopes, they might have the answer.

The welder´s goggles should be dark ones for arc welding. The glas for welding with a torch is usually not dark enough. I selected the cardboard sextant as the bubble horizon was essential for me and I didn´t find a plastic sextant that could be easily fitted with it. We don´t have an unobstructed view to the horizon were I live.
Looking cool when you make an observation makes some bystanders think you can achieve GPS-like precision. It´s essential to do the math when nobody is watching if you want to keep up the myth.
Using the sextant at least tought me some respect for the ancient navigators.

My brother, who worked at the Royal Ontario Museum when there was a solar eclipse in Toronto, told me that people were buying "solar filters" from some guy on the street corner because they were cheaper than the ones the museum was selling. <img src="/images/graemlins/confused.gif" alt="" /> Neither of us could believe (well, actually, we could - that's the sad part) that people would be so cheap as to risk permanent blindness for the sake of saving $10. <img src="/images/graemlins/confused.gif" alt="" />

Farley Mowat? Heck, if I'm gonna look dashing, I'll do it like Russell Crowe in "Master and Commmander: The Far Side of the World" <img src="/images/graemlins/wink.gif" alt="" />

Is this the same one?

Cheap_Sextant.

Not only should you use the arc welding hood, you need to use a shade 14 lens. You can still do damage to your eyes using a standard shade 10 lens that comes with most welding hoods.

Aardwolfe:

If you want to have the fun and would like to build one of these Sextants, then you should use the eclipse, or solar shades like the museums have to sell. They can be found at this website. Click on the link for free stuff and follow the directions. Not sure of international deliveries though.

If you want to do any solar viewing these glasses will help and for the price could be "modified" to use as filters in the Sextant.

There are other ways to view the sun, check out the info on this link.

Ray W. was right to direct you to use the Arc Welders helmet glass in #14 shade. You actually can buy just the glass for about $10.00 USD. You DO NOT need the entire helmet. Carefully hold the glass to your eyes. I have seen several amateur astronomers use this technique, with success, as they have using the more expensive solar filters for their telescopes and binoculars.

Good luck, let me know if you need further assistance.

Stargazer

"If we believe in absurdities, we shall commit atrocities - Voltaire"