My understanding is that the LT Cap provides only 2 things: aerodynamics, and extra length for stability at a faster twist rate. Its weight or composition is entirely irrelevant and the lighter it can be made, the more weight can be left in the actual slug where it can do the most good.

Injection-molded plastic should be fine IMHO.

I'll measure a cap on the weekend if no-one beats me to it. External dimensions only: I'm not pulling an original apart!