When our thermometer suddenly jumped into the fall zone, I realized the babies did not have warm enough sleeping arrangements. Tucking in a blanket was not going to work with two in the bed. They make these, of course, but I wasn't about to spend $40 on those when I can hardly take a step without tripping on a pile of blankets. (We used to go to a church where all the old ladies crocheted. D1 received a lot of blankets. And then D2 got some more. Mostly they have been used for dress-up.)
Instead, I figured out how to convert two of their existing blankets and some fleece scraps into homemade sleep sacks. Deducting the time for figuring out what I was doing, hunting up thread, oiling the sewing machine (which hadn't been done in years and I hope machine oil is all the same because all I could find was the oil for the hair clippers), and making mistakes, it's probably about a ten minute project.
One 36" square double-thickness cotton knit blanket
Two lengths of fleece, about 3" by 7". Make sure these are cut along the length of the fleece, so that the stretch is sideways--otherwise they'll stretch out of shape before you've got them on the baby. You might want to experiment to see how snug you want them to be.
Fold the blanket in half, right side out. Pin the right side of the fleece scraps to the wrong side of the blanket to form shoulder straps. Sew.
Turn the blanket inside out, and sew just up the side. The blanket is long enough and snug enough that they aren't able to kick out of it. (And D4 tries, oh, he tries.) But with the bottom open, you can slide it up to change the diaper if needed in the night.
To put it on the baby, just slide the baby in the top and slip the shoulder straps over their arms.
I put a zipper in the first model, and quickly realized it was completely unnecessary. If you do want a zipper, put the shoulder straps on either side of it so that it's down the middle where it might actually help.
We've been using these the rest of the week, and the babies have stayed nice and warm and sleepy.