How to make corners of video transparent

[peterkellner1]

I've been wrestling with masking and can't figure out how to do the effect I'm about to describe and similar to the picture below.

I have a background image that is 1920HD for example. I want to put a smaller video above it on the timeline. When I do that, it works perfect with the video playing on top of the 1920HD image. But, I really want to round off the top left and bottom right corners of the video with round corners.  I want the image below to show through on those rounded corners.  

Can I do this somehow in PR directly? I tried creating a photoshop image with a rectangle and made the corners transparent but that did not help me.

(Note: I tried the rectangle/opacity mask but that did all 4 corners, not just the two I want)

Thanks

[Ann Bens · Accepted solution]

Make a rectangular with 4 rounded corners in the EGP

Now put a smaller rectangular with no rounded corners over the big rectangular to fill a corner, do this twice.

Use this as a track matte or put the three shapes in a folder and mask video with shape in the EGP.