I think about a 2 inch sashing using your background color would look great....the X's would be floating.....I usually do my sashing by making rows.....and then adding the long sashing strip between rows....however, it can be difficult sometimes to have the blocks line up right when you do the long sashing strip between the rows and having cornerstones actually helps keep everything straight....so sometimes, I have done it with using the same color cornerstones as the strip, just to keep everything lined up right...I learned about this when I did my first big quilt.....just used long strips in between the rows.....and now I always giggle when I look at that quilt because the rows ended up being so wonky from each other....