Take a big-ish cardboard box. Stuff it with crumpled up bags, newspaper, rags, etc... Loose stuff, nothing hard that the BBs will bounce off of.
Put the whole thing in a big garbage bag (this helps trap stuff as the box/stuffing gets blasted).
Tape a regular sheet of standard letter paper onto it. Use marker/etc...to put dots/circles/etc... on it.
(hold an old CD on the paper and either fill the center hole with marker or a light shot of spray paint....makes perfect basement sized dots real quick)