I kind of like what you have going in the last picture because it looks like the "frame" is about the size of the top of the mattress. I think having that kind of border really defines the space for the beautiful star blocks. You could easily add an outer border to it to make the finished quilt the size you want. I have noticed that some of the most beautiful borders I've seen on the quilting board have been very simple with some great quilting to help give them "character".