Postscript - I did check that link to the polyethylene black tubes to replace Sonotube carboard cement tubes. The tricks would be how to make
the light traps on the endcaps. Then you'd want to put a bright battery light inside the capped tube in a darkroom to see just how light tight
this material actually is (as opposed to thick-walled ABS). My own tubes are made of noryl, which is a highly insulating but quite expensive
thermoplastic. And it would be important that the endcaps themselves be gluable; otherwise it would be difficult to install secondary light baffles. I also helps to have shallow ridges glued inside the drums, so that rinse water can get behind the print, though you can rinse in a big
tray afterwards if necessary.