I can understand basalt, as that is abundant on Mars. But butyl rubber, from where do you get that? Synthetize from CO2 in the atmosphere might work on Mars but on an Asteroid? But if you go through the route of synthetizing from CO2, why butyl rubber and not some other hydrocarbon?

I think that silicone rubber or silicone resins would be much better choices than a butyl rubber for such applications.

Not only silicone rubber or resins use much less CO2 and water for fabrication, most of their weight coming from quartz, but they can also be used in a much wider range of temperatures, compatible with that at the surface of Mars (i.e. including very low temperatures and high temperatures).

Silicone resins reinforced with glass fiber are a material commonly used where a wide range of operating temperatures is required, so I am pretty sure that they could also be reinforced with basalt fiber.