[AS2] Masking Duplicated Movie Clips?

Hey everyone, I need some help :D.

I used a tutorial from here, that creates snow, and changed it up a bit so that it now creates fizz in a soda. however I’m running into a snag.

Is there any way to mask duplicated movieClips so that they only show up in a certain area? Or some way to set parameters to accomplish the same effect? (i.e. I want the bubbles to only appear in the area of the cup, and not all over the place).

Right now I have just set X and Y values to do this, but the cup is angled and not square, so it doesn’t looks quite right.

I have tried setting a mask on a layer above which the duplicated movie clip sits, and also have tried setting the depth of a mask_mc so that it is well above any of the layers that the bubbles are generated on (by use of a for() loop), but nothing works.

Any help would be very much appreciated. Thanks.