1. Something strong. Seriously, it really depends on what fluids it is intended to see, what level of debris, temperature...
2. That would depend somewhat on the answers to #1, i.e. it may be material dependent, and to some extent it will also be size dependent. A spun 1" diameter bowl in stainless steel might sound reasonable (I'd think a machined part from forged or wrought stock might make more sense), but not for an 18" pipe line filter.

So, define the problem a bit better and maybe we can give you some better answers.

use steel or stainless or aluminum or titanium or engineered plastic or carbon fiber or fiberglass. The important part is the design stress. then hydro-test it to 4X, and consider the fatigue props. (stainless steel is the safest bet)

in otherwords, designing a 4000psi pressure vessel in not a joke. be safe, if it fails, someone could easily die.