RBD collision objects

[Shin-Kaiser]

I have an RBD sim where I use a pop force to push a thin rectangle along a plane in the X direction. The rectangle rotates from the added friction. I also have static collision objects to keep the box from veering off the X axis.

I then want to place objects inside the rectangle - so the rectangle then becomes the collision object for other objects. What's the best way to achieve this? Cache out the initial sim then use this cache as an animated collision object in ANOTHER sim?

or is there a way to get what I want from only one sim? is there way to have an active object to also serve as a collision object in one sim where there are also other static collision objects?