I saw a quilt at a show that had a wider top and bottom as well as a wider sashing right through the middle. It looked balanced. The quilter said it was for her 6'5" son.

If you want it rectangular from the start, don't make it 5x5. Instead, you could make it 4x6.

Beautiful quilt. I think I have that pattern, bought I when our DGD Zoe was born, but don't know if I'll ever make it.

If you can't come up with another row of blocks that look like they belong, go ahead and use the wider sashing in one direction. Or two rows of the narrower sashing with a contrasting band between them?