You want a 1/2 triangle for each of the 4 corners - you can cut this with the same template you used for your triangles, if it has a center line marked (don't forget to add the seam alllowance. As others said - this will make a recentangle, not a square. But should still work for what you are thinking. You could then add sahsing to each block to bring it to 8" square.
I think that "sashing" it with solid-coloured hexagons might be more interesting, however. Just scatter some solid hexies in around your patterned onces and see what pops out.