i think paper princess may be right on the math but it can be done with some work. here is an adaptation.
http://www.quilterscache.com/C/CrossandCrownBlock.html
or to adapt this block pattern you could make the center strip/square at 2.5"unfin(2" fin) and the other six sections are cut at 2.25"uf (1.75"f). this would get you to 12.5"uf block