Take a look at the homemade version of 4x5 tubes, it might give some inspiration: http://www.apug.org/forums/forum43/4...-your-own.html
That said, finding 7" interior diameter ABS pipe might be a challenge; anything that large will be for commercial/industrial use and probably sold in 20' lengths. And I think ABS pipe is slowly being phased out in favor of pvc. But you might be able to find scraps of pipe at a construction site.
If not lightproof, you could try coating with multiple coats of paint or even wrap wide electrical tape around the pipe. I assume you can use the tank in very subdued light.
Another thought would be the plastic containers for storing leftover food. You might be able to find opaque ones. Would likely require opening the top to change chemicals, so not truly "daylight" tanks.