i would sash the smaller blocks to make them all the same size- maybe 12 1/2", maybe larger- then i would sash the large center one to make it (divide evenly) with the smaller ones- (like maybe 18 1/2")
then i could use one of the smaller blocks with a couple 6" (6 1/2") ...or a strip 6 1/2"x 12 1/2"...what ever i liked as i was playing with it.
I love boxers...those pictures are WONDERFUL!..
start with getting your small blocks all the same size- then go from there...& have fun with it!