AV curtain material can often be had in significant yardage cheaply, since it's not in
demand much anymore. But being vinyl, it tends to outgas a lot. True studio flocking
is as black as something gets, but tends to attract dust and spider webs in a manner
which is difficult to clean off. A coulple layers of 6-mil black polyethylene works well, but tears easily and you have to patch over the staple holes or whatever. Better just to suspend a floating ceiling and caulk and paint it. Sounds like you're on
the right track, but again, beware of large surfaces which outgas and might smudge
your lenses and make the room unpleasant.