Gruesome mistakes find a home most anywhere at one time or another!
Hmmm, does it REALLY have to be square?? Can't it be longer than it is wide, such that they can flip the top up over the pillows??
I'm not sure you need to feel too bad. Think about it, and see if it's ok as it is somehow. Afterall, (as posted below!) you are the ONLY one who knows what it is "supposed" to look like.
Another thought, but don't take it too seriously, If this really bothers you, can you just add another border on the "short" sides, maybe the same color as your outer borders so it won't scream at you, to make it more square if you think it needs to be?
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