Congrats on joining in with the saving animals missions! Pilots N Paws is a great organization.
I use a collapsible/foldable crate in the back of our -6, sitting on top of a plastic cover in case of "accidents."
Sort of like these: http://www.amazon.com/s/?ie=UTF8&key...l_8vlot3h0r4_b
...just get the size that will fit back there.