recent example of transmissive plastic
http://www.popsci.com.au/technology/...-boston-bomber - optically opaque material strong enough to use as a boat cover was still fairly transparent in thermal IR and allowed imaging through it.
also seems that bubble wrap may be worth a try
http://climateandstuff.blogspot.com....in-camera.html
cellophane?
http://www.coloss.org/beebook/I/physiology/5/2/2/4
Tupperware possibly?? might be worth getting an IR non-contact thermometer and doing some tests around the kitchen.
maybe
http://www.edmundoptics.com/optics/w...lated-products (this has a polyethylene spectrum)
or just buy a cheap PIR intrusion sensor and use the window from it