Has anyone used anything like this for a project box?
I'm guessing that fumed silica (Cab-O-Sil) has been added to increase the stiffness, or perhaps they somehow managed to embed stiffener sheets effectively making it a composite layered material.
I'm interested because very few plastics are UV stable, easily glueable, tough, heat resistant and cheap.
Silicone rubber should in theory be compatible with silicone rubber based adhesives. I know from experience that regular silicone sealant intended for windows has excellent adhestion to FR4, so conceivably waterproof through hole connectors could be mounted on a stiffner plate and made into a FR4-silicone rubber-FR4 sandwhich.
Typical picture
https://kmartau.mo.cloudinary.net/805d8638-c8a9-4c1e-86fc-a46802c76a43.jpg