I made one up as you did, just dimensions of the sides and bottom, and instead of the sides I used four blocks I had made that I didn't need. I added a quilted rectangle for the bottom and lined the whole thing. Added another band to the top and the same as a facing inside. Sketched it all out on graph paper first and it went together lickety-split.
I gave the bag with the baby play mat to my niece and she can't say enough about it. Here's what it looks like.
Playmat and Tote
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