Spline for flexible plastic windows

by Barry H
(Hemingway, SC)

Trying to replace plastic in a florida sun room. Tried round splines, but I can't get wrinkles out of plastic. I think the problem is that I need new original spline. I have attached a picture of the spline. Can you help me? Any information will be appreciated.

I was sold flex-o-glass 4 mil plastic to go in it. Is this the correct mil plastic to use?