Great - thanks David! And I'm thinking of Jokers specifically. So you'd just put the scrim in between the lens and the barn doors?
This is a constant problem with Joker fixtures. With how convex the lenses are, they take up most of the room for scrims. Also, constant contact with glass at those temperature can cause them to break (we broke 3 lenses on the show I just finished). You can fit a scrim behind the lens as well if taking off light can't be done any other way. Otherwise, ask a helpful grip to fix the problem for you.