I see the problem. Which shape do you want for the gap? You could fill the gap with overlapping of several components or you could put some kind of keystone on it.
I want a shape like the one I put into the gap in the 2nd picture, but with a thick sides that correspond to the thickness of the "hollow" shapes.
I've tried various piecemeal fixes, including using the "top resizing only" feature, and I haven't been able to come up with an attractive piece that doesn't look, well... rigged together out of scraps.