As far as I can remember, I´ve never ever took part on a single keiko without an eye protector inside my men.
Unfortunately, some greedy kendo shops dare to fetch about 15€ for a single, clear piece of plastic, duh.
I finally tried to copy these by myself, and I think it worked pretty well:
What do we need?
Polycarbonate (PC-Plastic) 2mm (about 17€ for 40cmx50cm - about 9 pieces when cut)
(DON`T use Polystyrene or so called "Plexiglas" which will snap and break easily. Polycarbonate is much more stronger and prone to breaking)
A scriber.
Pliers.
Deburring tool or exacto knife.
Step 1.
Lay an old visor onto the plastic for tracing.
Leave the white foil on, important!