Certified solar film is sold in sheets so that people can make filters for instruments no manufacturer sells one for, and that is a legitimate and common practice. It is also the one place where do it yourself and solar viewing overlap safely, which is worth separating clearly from making eclipse glasses at home.
Why the sheet is different from household materials
Certified solar film is a manufactured optical product specified to block infrared and ultraviolet as well as visible light. It is the same material used in commercial filters. What you are making yourself is the cell that holds it, not the filter itself, which is why this is safe and smoking a piece of glass is not.
Building a cell that cannot fall off
The cell needs to grip the tube firmly enough that wind or a knock cannot dislodge it while the instrument is pointed at the sun. Card and foam board are perfectly adequate materials; what matters is a snug fit, a positive stop, and no path for direct sunlight to leak past the film into the tube. Many people add tape as a secondary retention, which is sensible.
Inspecting the film before and after every use
Hold the finished filter up to a bright indoor lamp and look for pinholes, scratches and creases. A pinhole in solar film is a hole in the protection, and film is easily damaged in storage. Repeat the check every session rather than once at build time, because the failure mode is invisible until you look for it.
Questions people ask about solar filter
Is a homemade solar filter safe?
Made from certified solar film in a cell that grips the tube securely, yes, and it is standard practice. Made from household materials, no, never.
Where does the filter go?
Over the front aperture of the instrument, always, so the light is cut before the optics concentrate it.
How do I check it is still good?
Against a bright indoor lamp, looking for pinholes, scratches and creases, before every session. Discard damaged film rather than patching it.