Wow, what a find! I'd make it "stretch" somehow. Are there enough pieces that you can make 4 more blocks and use solids on each corner? Maybe put sashing between the blocks? You didn't find scraps in the scrap/fabric bin at the same shop, did you? There's the thing nowadays that borders don't have to be symmetrical. Move them over one space and fill in with another solid. Trouble is we all know that white is not always the same but perhaps you can find a similar color/print that would blend in nicely. Let us see what you decide. That is beautiful and carefully constructed by someone.