Welcome to the board. You will learn so much here. With 4 1/2” squares and scant 1/4” seam allowance (one thread less than 1/4” to allow for bulk when pressing) you will end up with a 4” finished square. Making 4 across and four down will give you finished 16” (16 1/2” unfinished). A lot depends on the thickness of your insert. You can make one and see how that works out. If it is too tight, try making your seam allowances 1/8”. You would have to decrease your stitch length and really line up the edges well so you are sure you grab all the fabric. You could also sew with the 1/4” seam allowance and add a border all around. Good luck.