I think you have a great start of your layout and probably you are going to have to have some filler fabrics in between. I feel white fabric would be nice to separate each block or 2 blocks together. I really like the idea "danydany" suggested with the watermelon block as a label on the back. This way you could make notes on the white fabric giving info of why the quilt was made. Good luck on your decision and please share with us your finished layout.