You might consider a square 3.5 gallon tank and cage system. For color, you might be able to find a stainless steel water jacketed system that someone is getting rid of for free/cheap. 3.5 gallons is a standard size for 8x10" hangers, 2 by 5x7", or 4 by 4x5". It been years since I've used tanks that size, but as I recall the basket held something like 24 stainless 35mm reels.
I have one cylindrical daylight tank that holds 12 35mm reels, so you can get them that big if you keep your eye out.