I have done this before with success using the box on both entities, both having their Polygon flag set. I believe the c_move function also needs "USE_POLYGON" flag if you haven't set it already. It seems to function Ok performance wise, being that the object is just a cube and not a complex shape.

So any entity would use its actual polygon shape or need to have collisions handled with the box model (polygon vs polygon collision, always).

I've been looking into other solutions so I wouldn't have to pass so much information from one object to the next (saving more memory as well), but the collision box model did work well for me.

Hope that works for you, good luck.