Edited by Alpenglow - 7/9/13 at 8:16am
We have a "pet guard"--the one designed for our car (by Subaru, we have an outback)--and it is screwed into place securely (installed by the subaru mechanic). I don't worry about cargo going through the guard (or the dog) because it was made for our car and professionally installed. However, I do make sure that any cargo back there is behind the rear seats (so it doesn't obscure my rear view AND so that in an accident it would (hopefully) stay low without hitting the pet guard.
After we got our mini van, one of the few things I don't like about it is for this very reason. I hate all the loose items. I might look into a pet gate, but don't think they make for mini vans.
When my son was an infant, DH placed a car battery on the floor at the base of the car-seat for transportation. I freaked out about it, he didn't think it was a big deal.