I also am a little confused by your wording. on the market is a very lightweight interfacing that you iron to the back of thing blocks. you hardly know it is there. square your blocks, carefully ! then add borders, etc to make to the size you want. if blocks are different sizes, you can add wider strips & then square all to the same size. you could also use two different colored fabrics in thinner widths. Sharyn Criag
has books out that are just great for fixing blocks.