So this is what I decided to do for the big black hole in GM's quilt. I had extra material from making the strips between the squares and from creating the bias tape around the perimeter of the quilt. I ironed on fusing material to make it more stable and cut out a heart. Afterwards I cut little slits around the curved parts of the heart, folded it down and stitched around the perimeter to prevent any fraying. Then I pinned it over the big black hole and stitched the heart onto the quilt.
The final layout of the quilt with no hole. Is it even noticeable?
And now it's officially GM's quilt. The final quilt lays nicely on a queen size bed.