If you want it to last, cuben fiber would be the way to go. Find someone that makes either sails or backpacking tarps and/or hammocks, they might have a small strip you could use. I'd use the thinnest vinyl you can get, although putting it on a cutting mat and using a sharpie holder would be even better. (Didn't think, cuben is mostly transparent, would only be readable from one side, but so are regular air banners.) It is 2 layers of mylar with kevlar fibers sandwiched between. You can poke little holes in it, but you won't rip it. You could glue or tape a very thin dowel piece on the trailing end to keep it weighted a bit and vertical.
If you couldn't find cuben in time, maybe a strip of tyvek? very light, yet stiff enough to trail straight without too much flutter. I'd even try cutting the tyvek itself (on a mat) so the letters show through.
Not sure if either would really work, but it's what I'd try first. Hope it helps, take a video and share it here, cool project.
***I just remembered some fabric that I have upstairs, it's a very thin ripstop nylon (the parachute pants reminded me.... *shiver)- something like that could be cut with a hot knife and rigged and weighted with VHB tape.
