I made one several years ago. I had the same problem, so I solved it this way: I used a medium color sashing, all the same width. Then, for the blocks that were smaller, I framed them in a dark fabric to make them the same size as the others. It's very similar to what katier825 did, except my sashing was narrower. I had 25 t shirts, and made a king size quilt. I loved the way it turned out.