I was not good at geomentry and don't know the names for shapes except squares and triangles. That said.
I have had the same problem and I pieced it in the center with the shape that looks like a triangle with the top cut off. Depending on the design of the top you could repeat the shapes to piece the border. For instance if the center is squares you might do some smaller square blocks to add length. Or some diagonal pieces in the center. Or two or three narrow strips in the center like rain fence.
A picture or name of your design would help.