Asymmetrical transparencies

A little situation currently bugging me: I want to make a transparency asymmetrical, namely rectangular, not symmetrical (square). But the only options offered by CDraw are symmetrical, as in squares or circles (radials). I applied a square transparency to the checkered background (but not the oval foreground) in the attached image and indeed got the shape of a square, but that's not what I was after—the result is much more transparent left and right than it is top and bottom. Any ideas on custom shaping transparencies?