Use the ABS (black pipe) rather than PVC (white pipe). If you take a small flashlight, you can see how much light is able to leak through PVC. It's not a lot for thick-walled pipe, but it is significant. There's not a lot of price difference between ABS and PVC for these type of projects, so there's no reason to buy PVC and try lightproofing it in other ways.
I've used ABS pipe for other photo projects (such as a spectrograph, where light leakage would compromise the integrity of spectra captured) and had success.