I recently saw a technique that was used for sewing different sized blocks from a panel into a quilt top. It used a common sized block (I would use a regular quilting cotton) that was a good bit larger than the block to be featured. then the featured block was centered on the common sized block and edges were folded over and stitched all 4 sides. If you used 505 spray to center and hold the slippery blocks I think that would work. It gives a built in sashing around the featured blocks. They would be stablized from stretching by the cotton larger block. Someone on here will remember and link to that technique I am sure~