they hold up just fine. you don't need that tool to make them, either.
cut a batting square. cut a backing square 2" bigger. use the first inch of any ruler to line up the batting on 2 sides and voila! it's centered.
also, although sturdiness isn't really a problem, if it worries you anyway, just topstitch an extra strip of sashing between the blocks.
zip ... zip ... zip ... yooze done. :wink: