Despite how the picture looks, the rectangle is not crooked. There are plenty of other (minor, I hope) flaws in the wall, but that is not one of them.
Here is a close-up of the frame:
And here you see the new chalkboard posing with my great-grandma's chopping bowl. We don't use it as a chopping bowl anymore. It's more of a mail/rubber band collector that I try to keep empty but it never seems to stay that way.
This is what I learned from this weekend's chalkboard adventure:
1) If you don't have painter's tape, electrical tape may not be the best substitute. It's pretty stretchy, and is not easy to get straight.
2) Cutting a 45-degree angle is really hard!
3) Cutting edges to be the same length is not too easy, either.
4) But that's OK, because it's pretty hard to get a perfect rectangle on the wall.
5) If you find yourself whispering, "it is what it is," as you near completion of your DIY project, it is a pretty good indication that some (or most?) things did not go exactly as planned.
6) Most project flaws are best kept a secret.
And here's the closing image, just because I like the giant bright space with the huge chalkboard wall. Painting an entire wall is one way to avoid that nasty task of painting a rectangle.