What I would do is measure the smallest square and the largest square. If the difference was for example 1 inch, I would put a 1 1/2" border just across one side and either the top or bottom then square up the blocks to all the same size. Do that the same on all the blocks. All tops or all bottom, all left side or all right size.
After that you could add any size sashing you want to between the blocks. The border on only two sides would look like it was planned that way.
You could also put the border around the whole block, trim to the same size and then sash.
I think the first option would be more like it was planned that way and not a mistake!
:-D :-D