The iconic picture that says Hollywood - Marilyn Monroe standing on an air vent...No, no, no - enough of that. Expunge that thought*. I mean the iconic picture of a Klieg light on a stand with barn doors open on the front of it.
Those elephant-ear flaps at the side of the light are duplicated in nearly every lighting manufacturer's catalog, whether it is studio flash like Elinchrom or Profoto or strobist gear from the little Chinese shops. The method of attachment is different in every case but the basic form is the same - four plates that fold into themselves set round about the light. They are different sizes and materials but they all have one thing in common - they are too small.
Shock. Horror. Manufacturer's representatives reel back aghast. Someone has discovered the truth. Dive for the foxholes...
I use Elinchrom lights in my studio and have a set of barn doors for the 21 cm reflectors. They are well-built and double as gel holders, but they do not restrict the light enough to make themselves useful. The flash tube ring is big enough that it always looks past the door flaps as soon as they are opened even a peep.
Don't accuse Elinchrom - the other people's offerings are no better. There is always some place that the light leaks out on all the barn doors.
You see, what we didn't really see when we looked at the classic Hollywood doors is the fact that they are sitting on big - focussed - lights and the doors are really massive. And the film crews supplement them with flag flats and drapes and all sorts of other light restrictions. In the end I decided to follow them.
I went to a firm that makes display banners. They sold big sheets of Foam-Core board in flat black, about 3.5 feet by 5.5 feet. I used gaffer tape to join three of these together on the long edges and the whole flat can then stand by its own effort like a dressing screen.
When I need to restrict light, once the barn doors are in place, I prop the flag flat up around the light stand and I can direct a blast of light very accurately. Actually, I have two of these flag flats and they are essential to dramatic lighting. Like most of the best studio gear, they are home-made.
And there is no sense locking the barn doors when the light has flown...
* Haven't expunged it, have you...?