I've done this a few times. Choose my border fabric (at least the inner border if you're doing more than one) and cut a strip of that fabric to put on the top and bottom only...then make up another strip of blocks to add to that (again on top and bottom only) then put the border around everything. For yours, you could do smaller versions of the stars that you've done in the middle, or so some other simple design...just straight strips of the colors used in the center would also work.
Another option is to simply put the border on but make sure the top and bottom strips are wider than the side strips. I've done that as well - don't like it as much as the first idea, but it will work.