Is there any supplier of IR cut filter material? I.e., material that will block IR as opposed to allowing it to pass through? Preferably plastic, as I would like to be able to cut and shape it as appropriate.
Try Edmund Scientific/Optics for prototyping materials.
IR (and IR/UV) blocking filters are commonly used in astrophotography (since most telescopes with refractive elements are only corrected in the visual range, though there are some corrected out to nearIR/UV). I'm not aware of any plastic versions, though, but you may be able to get unmounted glass ones. A web search should pick up several suppliers - I know Baader Planetarium do mounted versions, not sure about unmounted, and I.m pretty sure all the other astrophotography filter manufacturers will do them too.