Several Install questions

In next several months im going to be installing. Im wanting to build a box for 2 18"s and then build a cover where i can run one 18" for competition (street C). Anyways I drive a 2001 alero, what is the Peak HZ for the car? also im going to be sounddeadning how many sqft should i get to cover the car twice.. Yes i already know i will not have a trunk anymore. But hey im a bass head.. thnx for ur time