I've been using mylar for various things, adding extra layers of glazing cheaply. It's dead clear. It does deteriorate under UV and needs to be protected from UV.
So far I've been using Sun Tuf, which has a UV shield on the top, polycarbonate needs to be protected also. What I don't know is what other plastics/glazing blocks UV. Acrylic and PVC as well as polyethelene are widely available. I'm fairly sure that the poly is transparent to a wide spectrum. I think vinyls and probably most plastics pass UV, but I really don't know and I can't find good data on this.
Any help?
Jeff