Do you have a picture to post so we can see the issue? You can iron the open part of the seam line to get definition. Turn right side out. I use steam a seam to close and then top-stitch.
You can use batting and cut a little smaller than placemat. I like fusible batting or just fuse the batting. Stitch the two fabrics WST, turn and top stitch.
One last option for you. Grade out the excess batting around the seam line before you turn.
Hope that helps!