The clips used to hold the corners are taken from an old Kodak film processing hanger. I found a few of them at a flea market years ago and finally put one of them to use.  There were four per hanger and had to be heated with a torch do de-solder the connection. 

The illustration is fairly self explanatory, but what is not shown is that each of the clips is allowed to swing from side to side. This is significant since it allows the tension to stabilize uniformly when the knobs are tightened.  When the neg is tight and ready to go, a light tap with a finger will produce a note like a tympani.
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