I don't have any advice to give you concerning its construction, having never made a tail, but I can help you find advice! Most of the people who would be able to give tips about this kind of thing would be looking for your thread in the Cosplay Construction forums, probably in the Props/Accessories subforum. If you post there, I think you'll be way more likely to find the help you need!
Scarlet Witch (with Quicksilver), Dagger (with Cloak, 2013 Ultimate editions), Stahma Tarr, Pyrrha Nikos
Chosen, In Planning:
Red (with Subject Unknown), Tempus, Sahkmet (with Inanna)