Hmm, this could be a tricky one. One thing you could do is create a sphere brush and tie it to func_physbox. Make sure the flags "Start Asleep" and "Generate output on +USE" are checked. Place it on a floor that's on a small angle, just enough for it to roll, then place a small clip brush to block it from rolling any further.

What this will do is, one you press the USE button, the boulder will wake up and roll until it hits the clip brush and stops. As for closing it, well that's a bit more tricky.

It seems complicated, but this is the only way I know of in order to make it have that realistic "roll" movement.

Turbosmooth dat shit.

Ehh thats quite confusing if someone made a prefab :P