Using the Titler to draw masks -- how put it together?

For lack of a rotoscope function in PPro (16 point garbage matte being the limit), I tried using the Titler to draw a mask, but can't get the transparency to work.

The base clip is a static skyline shot.  Using the Titler, I created another title clip, using the base clip, with a mask drawn around the sky.  The idea is to make the area enclosed by the mask transparent, so as to replace it with the sky from another shot -- and retain the rest of the base shot, which (on the Titler) is the area outside the mask.

But, using these 3 clips on 3 tracks, and the track matte filter, I can't get it to work.  Any ideas?