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.