I dabbled in plastics research many years ago as I needed to find a cold weather and simultaneously Hot weather durable type of material and found out that there are literally 100s of choices.

At the time my plastic of choice turned out to be HDPE - High Density polyethlene -- good to Minus 100 degrees and hot attic temps exceeding 200 Degrees.. Ps HDPE is "Bucket plastic" - Cheap Garbage & Kitchen Basket type Material.

I would venture to say that the clear airtites and directfits are of a regular grade acrylic or "optical grade - Xtra clear acrlyic" as used in some of these "New" Xmas tree type ornaments Angels, Santas, etc. As short as 15yrs ago most of these ornaments would yellow soon after purchace, now they don't, and optical non-yellowing plastic is the new normal.

Also being a coin collector the soft flips are bad..common knowledge among coin collectors .

Pss all plastics are on a path to destruction, they loose their elasticity, volume, & shape due to many variables - Uv rays , heat, cold, "chemical reactions internally" that start and continue to happen all the way to total degradation.