You could try this:
Taskmaster BLACK FELT ADHESIVE VELVET ROLL. It used to be available at Bunnings, but they don't seem to have it anymore.
I'd be nervous about putting anything with out-gassing potential inside my OTA. Richard's advice might make me re-think flocking the interior, though.
I use EVA75 foam (6mm) as a dew shield for my C14 - it's lightweight and naturally matt black. My next move is to re-do it with 1.2m wide foam to insulate the OTA as well (with a one-piece foam roll) in a bid to stop tube currents. Alex (of Astro Shop) has been talking about that for a while, so I figured I'd give it a try. I don't know if that's any use to you.