After my original design was field tested (with overwhelmingly positive feedback) I've put some more time into the model.
This new version does not require any additional modifications, no string needs to be attached (pun intended), no holes need to be punched. The transparency paper is held in by model's clips (you do need to crease it 1cm at the top), the visor itself is held using tension, although I've added hooks in for an elastic band for even better tension on smaller heads.
In short, my faceshield:
To assemble take a sheet of A4 or Legal transparency film, lay it down horizontally. Gently scar it about 1cm (half an inch) from the top with an exacto knife (optional). Fold it to make a crease down the line you made, put it into the visor with the crease facing the inside. Pinch the corners with your fingernails to create a tiny crease at the 45° angle on the edges, just to secure it in place. Optional: Add a piece of sponge or fleece on the part that touches the forhead using a method of your choosing, for additional comfort. I've received no complaints either way.