Making the inner border:
The key to this border is allowing plenty of room on each side of your squares-on-point so as to not cut off any points when sewn together. I love allowing lots of room there and trimming the strip to 3" will give you that extra room! The top and bottom each require 21 units and 23 for the left side. You will only need 18 for the right side if you add a snappy label for the front; a tree, a heart, a star, or whatever you decide. Kids love all those things and it adds a nice touch complementing the small blocks. Add your 3.25"x10" muslin strip for the label area.
Before trimming your strip sets to 3", you will be adding an extra white square to the left side of the strip and to the right side of the strip. This will be your 'cornerpiece' of the strip.
Just make sure you align the ruler's lines with the center line going through the middle of the printed squares and it will come out flush no matter how much bias is at play. Starching may be done to minimize the bias stretch, but I eliminated that step and it worked out great. As long as it's cut right/sewn right, it will work out right.