Yes, I made some film sheaths, in the 3A size. You need some thin aluminum. In the US hardware stores carry K&S brand sheet metal, don't know what the sources are in the UK.
You obviously want one as a reference of how to bend them.
I notch the corners to size, but leave the sheet oversize. Then bend up one side, past 90 degrees of bend, so that you have a solid bend. Then trim it to the proper size, then finish the bend.
It will probably take a failure or two to figure out the right place to start the bend, as the "slot" will get smaller as you bend the metal over.
Test the size with a genuine sheet of quarter plate sheet film.
You should paint it flat black when you're done.