Just by chance, I found a black stretch cotton bustier at a yard sale for $3- just what I needed as a costume piece for Steam Con. It has a side zipper, and while the bust fit, I could not get the zipper to close over my lower ribs or my waist. Not by a long shot. Yes, I am that oddly shaped.
So I did what any frustrated person would do: I cut it down the back.
Then I took another piece of black cotton that was the height of the back, plus the length of the facings and a 1/2" turn under for the facing hem. I pinned and stitched a piece of black ribbon with white dots on it, similar but not an exact match to the ribbon that goes around the bustier under the bust.
Then I pinned the new piece of fabric to the cut open back, right sides together, and sewed them.
It looks terrible now- the black doesn't match and the ribbon is different. But bear with me.
I folded along each of the new seams and pinned them wrong sides together. Then I stitched an inch in from the new seams, doing it twice for strength. There were now two flaps sticking out of the back.
Next I put four large eyelets/grommets whatever you call them on each flap. I need to get some grosgrain ribbon for the actual lacing, but this thin ribbon gives you the idea. Voila! Mock corset top!
For the finishing touch, I sewed some ribbon roses that R sent me to the front.