I have been researching lenses for thermal imaging cameras recently because one goal is to put a camera outdoors in a weatherproof enclosure streaming to software looking for heat signatures of wildlife crossing in front of the camera. But because traditional glass and plastic materials are opaque to LWIR radiation the thermal imaging camera senses, other materials must be used. Expensive materials! Searching for lenses made with the materials Flir recommends in the data sheet such as Germanium and Gallium Arsenide return results that cost far more than the Lepton itself. Many of these lenses also can magnify or change the focal length, capabilities that may not be needed for simple applications where all one wants to do is protect the sensor.
Further research took me to the EEVblog forums and some of the incredibly smart members there. They were discussing this stuff way back in 2013 and 2014 if not sooner. There I learned that materials like Polyolefin (shrink-wrap) film and even common kitchen materials such as cling wrap or plastic sandwich bags are very transparent to LWIR.
A very quick experiment was somewhat promising. My "super scientific" setup is shown below.
The camera can still take a good image but computes a different radiometric temperature. I suspect reflection from the camera's own internal heat from the plastic bag. The more expensive lenses have anti-reflective coatings.
A single layer of cling wrap resulted in a slightly smaller difference, less than 4°F.
Obviously one wouldn't want to use cling wrap to protect against flying debris but with some kind of shroud or other structure could be quite useful for an outdoor thermal camera.
I didn't test but read that Flir sells Polyolefin protectors. I plan to obtain and test some Polyolefin sheets as well, preshrinking them over a frame that could be mounted in front of the sensor.
Finally, an interesting link I found while researching.