I have a little bit different suggestion since you have plenty of fabric. I really do love the look without the sashing, but also think the blocks would shine with some sashing, so how about this:
Make a center panel without sashing, add a border of a width to bring it up to the size needed for the blocks with sashing and add a 3 block wide border of blocks WITH sashing around the center panel.
You could even use cornerstones in the first border and the sashings to add an outgoing effect of squares in each corner.