I have been looking for tubes or drums also. I plan to develop film and paper, up to 50 x 65.5. I may have found a solution. Tonight I stumbled on the ceme-tube HDPE plastic concrete forming tubes. They come in 4 foot lengths and in diameters of 8, 10, 12, 18, 24 and 30 inches. Google "ceme-tubes" for information. There is even a video on Youtube showing how they connect. They have optional caps which look like they will fit one end of the tube. According to the web site, the tubes can be cut to length using regular power tools.

I plan on ordering one with a couple of caps to experiment with. I was hoping to attach a Ilfochrome light trap to one of the caps for adding chemicals to the tube without having to remove the cap in between chemicals. White Cap construction supply lists the tubes for sale on their website--and they have locations near me.

I assume they are light tight, but I'll check this once I have the tubes.

The smaller tubes look like they would work for your situation.