Jffe (you got some interesting image fx filters), thanx for the idea, but I actually found another way around the problem. I think I tried what you did but I didn't like how the cheerios were shadowed around the edges. Unless that actually is a good idea...
Anyway, I ended up using my handy "circle perfected" snippet to create a "size 75" circle, using that as the new "set alpha" for both the height and the color, with another size 87.5 for the color to keep the background blank.
This update should come tomorrow