The solar filter material we recommend is AstroSolar Safety film, http://astrosolar.com/en/ , which is a 0.01mm thick material. It doesn't matter if it has little waves in it as mounted, but you don't want to stretch it. Thus, print the two pieces, make sure they fit reasonably (remove any print defects such as stray hairs), and then spray contact cement on the side of the threaded half that was printed facing down. Once that has partially dried, simply press that onto the film. The film can then be trimmed to the edge of the part. Finally, the second part is fitted over the film and a soldering iron is used to permanently weld the two pieces together. This is essentially the same process described in https://www.instructables.com/id/Safely-Shooting-the-Sun-With-the-Canon-PowerShot-S/
Note that the 20170816 version is now replaced with a 20170820 version that has thicker sidewalls on the grip for larger-diameter filter holders. The earlier version is still perfectly usable, but the extra rigidity helps.