To make the blanket more square…. I modified the last 10 rounds.

Last 10 rounds:
Knit round: (k1f&b, knit to 1 before the marker, k1f&b, slip marker) repeat around the round. This puts all increases on the knit row, 2 at each corner (one on either side of the marker).
Purl round: purl around (no increases).