This was actually faster to make than I thought! I made a minimalist diaper bag that we call the One Hit Wonder. It holds one diaper and one travel-size package of wipes; no more. It zips, and has an attachment loop. We grab it when we go to the store; we don't expect to need a diaper, but if we do, the One Hit Wonder has enough (one) to get home again without screams for a diaper changing.
The cotton quilting print I used had brightly-colored fish swimming on it. I didn't want the fish to be upside-down on one side of the bag, so I cut two pieces of fabric 5" x 9" for the side panels. (Karston still wears size 3 diapers and still weighs just above 18 pounds. You might need a larger size for a larger child.) I ironed interfacing on the back before cutting to give the bag some shape, to keep it from being floppy. It's often easier to work with interfaced fabric because it doesn't stretch while you're stitching.
For the zipper, I cut two 1.5" strips about 30" long, no interfacing. Using a very long, very loose stitches for machine basting, I sewed down the middle of the two long strips with right sides together. Next I ironed the strip so that only the right side of the fabric was showing, folding at the stitching. From my zipper spool, I cut off about 18" (for 15" of zipper use). I stitched the sides of the zipper face down onto the long strip, and then removed the machine basting stitches. Voila, perfect zipper casing! Next I sewed back and forth to bar-tack both ends of the zipper. Finally I stitched the long loose ends together where I had removed the basting but had no zipper, and bar tacked farther down the zipper for the little discrepancy area where I couldn't replace the basting but didn't need an opening for the zipper.
I carefully pinned the zipper track to one bag panel (wrong sides out, right sides facing), and stitched it so that one long side zipped open with equal amounts of zipper on the two short sides. Then I stitched the other panel to the other side of the zipper track to make a short rectangle.
And that's my One Hit Wonder, a single-use diaper bag for those short trips from home!
Then a few days later, I took some skinny fabric left over from cutting the zipper track, made a loop, and stitched (bar tacked) both sides of the loop down near the zipper slide (when closed). The attachment point is a lot like a belt loop (also called a carrier). Now when we're clipping toys to the car seat or the shopping cart cover, we can also clip the One Hit Wonder on the menagerie.