Some thoughts.
Make 4 rows, with 4 down and each corner is a blank for either a fabric or a HST's or???
blank, block, block, blank
block, block, block, block
block, block, block, block
blank, block, block, blank
Use a sashing to put the blocks together with a different solid color at the intersections and at each corner.
Add triangles on each size to make the blocks, square in a square, then they will look like they are on point. Then use a sashing to highlight the blocks.
An important part is what size do you want this to be finished as. Do you want borders or is this a standalone piece. Four 18 inch blocks is 72 inches square without borders. If you add 1 inch sashing and edging, then you increase the size 5 inches.