The simplest solution that would provide the same level of processing quality and ease of use as a 35mm steel reel is to have a machine shop cut some down and reweld to size.

Now if you took that a step further, a cut down reel could be made adjustable. Cut them apart close to one spiral, weld small stainless tubes with an ID slightly greater than the wire size of the reel onto each short stub, tap two of the tubes near their outer ends for a set screw. Insert the longer prongs of the other reel into the tubes of the first one to the desired depth, and screw down the set screw.

It would take some precision welding, but that is not beyond the ability of a decent machine shop. Frankly I'd rather have reels cut down and rewelded to the exact width I need, as it is simple to do, but the variable width reel could be done.