It took a ripped seam, some precise measuring, double checking, and a sliced finger (rotary cutters are bloody sharp!) but it’s fixed and finished.
The cornerstones worked well. The finished top is about 70″ square, so just under a queen size, but it will make a nice bed topper. It’s up with Mom now so she can work her quilting magic. Hopefully you see an FO post for it soon!