Thanks for the replies! The foil sounds potentially messy, not to think of the glue! Thanks for stopping me from doing that And the about the dust, it might sound obvious but I didn't think of it, those lamps have been hanging under a studio roof for years so little cleaning won't hurt them.
Do you guys use the fresnels for lighting scenes? ASA 400 seems about the lowest you can shoot with a 2000W key. Wonder what lights guys like Vincent Peters use, he shoots kodak portra 160, but maybe those HMI lights are a different ball game.