I don't think it will fit perfectly with the 42" panel (as 42 is not evenly divisible by 4). If you want to make the proportions the same as the picture you showed, then the panel needs to be 48". can you add a 6" inner border to get it the right size?
when I do HSTs, I round up. I would cut 5" squares, make the HSTs, then trim down to the right size.