Question for Wayne: how do I “applied a negative mask to the affected area”?
Here’s an example, except the masked border is on the left.

Open the pan/crop window and tick the Mask box. Then select the “box” (I think V12 has this capability. If not, use the pen tool.) on the left and draw the rectangle around the area to be masked. Initially, it will be positive, meaning only the area inside the box will be shown. Select the negative icon, and then only the area inside the box will be masked. There are also options for feathering, but in your case that can be ignored. That’s it.
One other thing. Make sure you apply the mask at the beginning of the event, unlike the example above where it’s somewhere in the middle of the event.
IMHO, learning to use the masking tool in Vegas is well worth the effort.
wwaag