Welcome Rrainea ... from one Canadian to another!
So happy to have you join us here on the Quilting Board.
This is an lovely project for you to complete for your Mom.
And will bring joy to both of you.
With your measurements, adding on the 5th strip, would give you 22-1/2" x 54". Simply, it could become a bed runner, requiring very little "new" fabrics. Otherwise, my first thought rather than adding blocks, would be to find a coordinating fabric that you could run strips of it between the pieced strips you already have, as well as use as a border.
There are so many possibilities as to what you could do. However, it is a difficult thing for the rest of us to imagine what you really have as to what would go with it. If you could post some photos of what you have, I know there will be countless ideas come your way. Perhaps so many, that you may become confused!!
Good Luck!