Attaching Mask Dynamically to multiple instances?

Hi

I am attaching six movies via the attachMovie method. I have put all six of these instances into an array and everything’s going smoothly as all box’s move around in random places and bounce around around a little. Only problem is I want to mask all 6 of these instances via setMask so they are restricted to one area but can’t seem to figure out how to do that.

If that’s not possible I would also love to know if there is a way to do collision testing with them and then I could draw a border around the area I want them to stay in and have them bounce around that (I would actually prefer this)

Any ideas???

Thanks in advance