Hi,
I'm glad to see that you are designing a one of a kind quilt. I like to do that too or at least add a little something to make it my own.
In looking at your dimensions, I noticed that the yellow sides to the rectangle should be 24" by 3'. If I am reading this right, you have two 12" blocks to cover.
Have fun with your design and don't forget to cut that extra 1/2" for seam allowances.
:D Happy Quilting! :D