How to insert the "Frame Effect" in FCP, but have it only apply to a specific section of the screen.

I have been asked to take hundreds of videos which were edited in FCP for 1920 x 1080 HD format, and re-edit them for 720 x 1280 vertical format. Sometimes I can just adjust the scale, and do some Up/Down/Left/Right nudging to make things work. But sometimes, I really want to divide this super "tall" 720x1280 frame into two 720 x 640 "frames".

From a narrative point of view, I can make this work very well. But visually... it looks kinda crappy to just have two videos touching each other, edge to edge. I think I already know the answer... which is... "You can't." But I would like to be able to apply the Effect > Stylize > Frame effect in FCP, but have it only apply to the top half of the frame. I know that I could create a static .png file of a frame, and insert it whenever I need this. but I like how FCP lets you adjust the look of the frame dynamically to see if it looks better as a "35mm film stock" frame, or an old paper frame, or... whatever.

Anybody have any suggestions on how to do this? Because if you insert the Frame effect, it just covers the whole frame of the viewer. Even if you insert an Adjustment layer, and then try to adjust the Frame effect on the adjustment layer... it still affects all clips "below" the adjustment layer.