Waterproofing MLX90640 far infrared thermal sensor
There are a variety of common materials that are available to be used as enclosures  or windows for an enclosure i.e. Calcium Fluoride (CaF2, Fused Silica (FS),Germanium (Ge),Magnesium Fluoride (MgF2), Sapphire, Silicon (Si),Sodium Chloride (NaCl) to name a few.
There are also IR transparent plastics.
I had a similar application years ago and I started down the same path as you have, but when I started to go through all of the time in engineering and research I went with a commercially available IR camera with an IP66 rating.

If yours is a labour of learning, then Google "IR transparent materials" and in turn use them as a "window" to your enclosure for your selected sensor it would be the best place to start.

I looked very quickly on the spec sheet for your chip and I didn't notice what wavelength of IR it is the most sensitive to. You would have to ensure the "window" material is efficient in that range.