I made this a few months ago and find it handy for solar viewing especially when imaging. The one I made is not a thing of beauty, but is very functional.
It's to suit a 50mm finder scope.
The lid from a can of spray paint.
Some light cardboard.
On the lid cut out a circle .5cm from the edge.
Use the lid as a template, cut your solar film to suit and a circle of cardboard to suit.
Cut the cardboard to an O and mount the film to it. Glue this cardboard side down, to the inside of the lid.
To make the cap fit snug to the finder you need to cut a strip of 6"x2" cardboard glue this onto the inside rim of the lid leaving 1" outside the rim.
This should hold firm enough but for extra safety put a blob of bluetac on the finder to hold it in place.