I have two suggestions, perhaps more useful for the larger light traps in the camera than the film holders, but use them at your own risk. I have used each with some success, but others have suggested there might be issues with the materials fragmenting and causing dust in the holders. The first is to cut down the soft (loop) side of velcro tape. I did this on a 9x12 Rietzschel plate camera and it has not caused any problem, but it gets very little time out on the job. There may be more chance of breakage in heavy use. The second was suggested by John Wojtowicz, another PHSNE photographer, in his article on plate cameras for the PHSNE Journal, and that is to find an automobile restoration shop that does upholstery with velour fabrics. He says he has some picked out of a scrap heap at one shop that works on cars from the 30's and 40's and that it works great as a light trap. It's much heavier and more wear resistant than velvet.