Just pulled this one out of my head and from my history.
My grandmother would make bags to put shoes in. They are simple.
A chunk of fabric, torn to size sewn on 2 or three sides depending where the fold is. One more piece 3-6 inches matching the size of the opening of the bag. The variation is 2 pieces to match half the size of the bag opening. These are the casings.
1 casing: fold over short edge and stitch. Fold in half and press.
place edge in the middle of opening matching raw edges (this is where the bow will be tied). Stitch.thread ribbon through. Ribbon length just needs to match the opening of the bag and just a little more.
2 casings: fold over short edges and stitch. Fold in half and press. place edge at seam, matching raw edges, repeat.
thread ribbon through casing all the way around to where you started. Go to other opening and thread all the way around.
You have a self closing bag now.
Folded over top casing. Before stitching sides(fold at the bottom)fold over 4 or more inches of top of bag and press. Right sides together, stitch from bottom of bag to fold mark, repeat other side. Press seam open. starting at top of the bag stitch seam allowance, pivot down at bottom of stitching and stitch back up to top. repeat. Press 1/4 inch of top, fold the edge to the stitching edge and stitch around. There should be a casing.
Casing openings are where the ribbon, string or yarn goes.
However you can make a bag with a casing works. There is no bad bag because the pattern or plan is in you head.
Hope this isn't to hard to understand.
These are just two ways to add casings. The options are great with contrasting fabric. The bag tops can just be pinked and no casings used. If raw edges don't bother you, you can not hem edges. They bother me.
You options are great for adding the casing, contrasting fabric.

Clever idea.