I agree with adding a border, since you have already cut the squares and I assume you already have the forms.
My suggestion is also to actually measure the insert. Use a tape measure and measure all the way around the form. Add at least 1/2 inch to give it a little bit of give, as you want it snug, not tight when it's finished. Divide that number in two to see how big you actually need your patchwork to be.
I would not do anything less than a 1/4" seam, either.
My favorite youtube video about piecing is this one:
https://www.youtube.com/watch?v=HZ3vzVfrKIU
She has a whole series on quilitng:
https://www.youtube.com/watch?v=HZ3vzVfrKIU
bkay