Plastiglass

Plastiglass is an advanced polymer designed during the time of the Ancients, used extensively in industrial and scientific sectors. It is actually a development of military plastex (the same substance used to make super-advanced armors during the time of the Fall), and resembles transparent glass. Plastiglass’ rigid composition makes it ideal for use as a barrier in windows or even as entire walls.
Plastiglass is extremely strong and does not shatter when struck. It is almost entirely bulletproof, does not melt, and can withstand extreme temperatures and drastic pressure changes. Even more interesting is plastiglass’ ability to absorb physical blows by flexing like hard rubber; it instantly snaps back into place without so much as a minute vibration (and without distortion).
Plastiglass is durable enough to keep even the largest biomasses in place.