Looking at the pattern, each block has a different background fabric. So you don't have to commit to the "perfect" fabric. I saw a nice hand cut "selection" of fabrics that might do nicely (just read it on the board today).....I think it was Kansas Troubles. Let me find the link...
http://www.ktquilts.com/shop/KT-Prec...s-x2875673.htm
The owner cut these herself to make a pre-cut. Maybe she can make a pre-cut for you the exact size of your Caswell background block. what is it? 10.5", 12.5", 16.5"? Ask her, see what she says.