It seems that you don’t have a big 18 inch piece of the background fabric, but do you have enough in small pieces to be able to create a square? It looks as if you wouldn’t need huge pieces because the lovely Dresden covers most of the square. Alternatively (and I have no idea of a layout! All of the suggestions about going on point or alternating rows sound excellent), you could make a “reverse” square or two. What I have in mind is perhaps a pale green background that has a similar look to the current white background or you could go wild, with that dark green lattice fabric. You could change the Dresden’s petals to a paler palate. I