As an astronomer...............
Buy a sheet of proper Baader solar film. Anything else and you're risking damage to your camera, and possibly your eyesight. You can use a cereal packet to knock up a home made mount that just slips over the end of your lens, or if you prefer into a square filter holder. The slip over design is better for a large aperture lens. Every time before you use it hold it to a light source (a bulb, not the Sun) and check for any damage. A pinprick is enough to wreck the film and it's not very substantial, so store your filter carefully.
For visual observing be even more careful. Use proper eclipse glasses and again store and check them carefully. Also bear in mind that if you're using the eyepiece of an SLR design camera to line up on the Sun (rather than the rear screen) you are still effectively visually observing
For the forthcoming large partial eclipse with the Sun low in the sky it may be we get a 'natural' filter in the form of cloud or haze in which case a filter may not be needed. If you can't see or can barely see the Sun through the filter it's okay to take it off.
