Of course you can make it with a HST in the corner but I think it looks nicer done the traditional way with a Y seam. Y seams (AKA set in seams) are not hard at all. They just seem a little intimidating at first but once you have done 3 or 4 of them you will wonder what all the fuss was about. I wouldn't bother with a specialty ruler as the cut is a 45 degree cut and almost all regular rulers have the 45 degree line on them.