I think your sample looks divine! Great color combination!
Me thinks it is a 'rectangle' due to 'row gauge' that's not stacking up as tall as it should.
The only thing I can think of to correct the squashed appearance is to add rows inside the squares. Or maybe above and below the motif.
Ladies who are working on Sylvi, whose row gauge was squashed, but they needed the prescribed length, added rows to the chart for the BACK in discreet areas that wouldn't look odd.
Does that make sense? Here is the back of Sylvi:
Some gals added rows within the cable charts! So their SYLVI wouldn't be too short cuz their row gauge was squashed (too short).
Could you do that?
About the border: others in this KAL will know more about border choices.