The border can be anything that will compliment the squares. If you are going to frame them, you may be able to put a stabilizer behind them (like in a t-shirt quilt). If you have a good framer in town you might ask them for advice. I had one they stretched it onto a backboard and used the mat to frame it rather than fabric. But a friend of mine did it herself, put a fabric border and stretched that one the board and framed it. You have several options.