When I do the seam to sew the front of the pillow to the back, I sew over a folded piece of batt or stuffing in each corner. It makes it a little harder to push out the corners but the stuffing is right where it is needed. I put in a regular pillow form to finish.
If your machine cannot handle the bulk in the corners, hand sew a folded piece of batt to the corner seam allowance.