I keep meaning to install some cargo tie down points back there, but haven't so far. I can tell you about the time during Phase I when I had three 40 lb bags of concrete back there, and did a spin - and in the entry, the airplane unloaded and got a little negative. There was a loud "Bang!" as the ballast hit the ceiling of the baggage compartment! I thought I'd lost the tail - very un-nerving, and I never had heavy ballast back there again without something at least wedged in (foam works) to keep it from shifting. Probably my scariest experience in Phase I, and my own fault - IO should have thought it through better!
(Of course, foam isn't going to secure it in a crash! I always buckle the rear harness when there is baggage and no one in the rear seat to keep stuff at least contained....)
Paul