usually some sort of soft stuff is attached to the walls of a bass reflex

Also, would i beable to put a horn on the inside of the cabinet?

a horn is a type of cabinet rather than an add on ....Like the Pi6 or Pi7

And, becuase i've never done this before...i don't undertand all the instructions that came with the email...would someone be able to explain them to me?...thanx

Pi is famous for their customer support, and others on this forum are happy to help