Back in the day we would fly our Aztec in "training mode" which is two upfront and full fuel with a 70lbs sandbag secured in the luggage compartment.
You just can't simply drop a weight in the back and call it good.
Pa28 lugage compartments have "walls" that are even less usefull then wet cardboard to contain anything.
Make sure you secure any weight to part of teh structure/frame so it doesn't go flying around in turbulence or during a bad landing or even a go-around.
That's the last thing you want, a 50 lbs weight rolling into the tail.