Here's another idea, perhaps much easier to implement for short lenghts of film than the PVC drain pipe processing tank I previously posted. This is from Popular Mechanics, October 1947. This article was written with movie film processing in mind, but you can adapt it easily. You'll be doing tray processing, but the methods described are simple to implement.
You also might be able to adapt a Morse G3 movie film developing tank. They come up on ebay all the time. They are designed to handle both 16mm and 35mm film. Since the film is spooled from one core to another, back and forth, it may work with other widths of film up to 35mm.
You might also talk to a machine or welding shop in your area about making you some custom width reels. An extremely easy way to do it would be to cut down and reweld some 35mm stainless reels. They would just need to know the precise spacing you require between the spirals. I doubt this would cost more than $25 per reel.
If I needed custom sized reels for short lengths of film, I'd probably take this approach. I believe I could do it with my chop saw and MIG welder, although it probably wouldn't look as good as what a machine/welding shop could do.
Maybe the S.K. Grimes shop, who I believe is an APUG supporter, could do this.
They worked well in the Kodak tanks, but not so well in others that didn't have the same dimensions or the same "stub" at the bottom of the tank.
I stopped using them regularly because the Kodak tanks don't permit inversion.
For the apron to work, the film width must match the apron width. The ridges on the apron are to suspend the film so it does not touch.
To solve your problem I'd just modify some reels. Plastic reels and tanks are cheap or free. I use a modified Jobo reel for 8.5x11 (Minox).
Thanks all. It is amazing how seemingly simple question can end up being so difficult to find a definitive answer for. I still think that 'standard' click steps on adjustable reels should have been abandoned in favor of odd sizes never being a problem. But Americans always like 'convenience before universality' so the marketplace disqualifies my opinion.
I liked the 1947 Popular Mechanis article and others here have contributed well to this quest. Thanks. - David Lyga.
I wonder if you could rig up custom stops for an adjustable reel using stainless steel pipe clamps or something similar. Even nylon cable ties might work.
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.
You can make a long processing tray (or tube) with a trip to a hardware store for some PVC pipe, and develop the long roll like sheet film. To make the tray, cut the pipe into a semicircle, and make "feet" for the ends, so you don't have to hold it level the whole time.