When I mounted counted cross stitch I used masking tape at the top to hold it still initially, then did the criss cross "lacing" with thread to get it to fit snugly from side to side. Then removed the tape and repeated the "lacing" to get the top and bottom snug as well. Since my margin was only a couple inches all the way around (plus the half inch turned under to prevent raveling while stitching) there was quite a bit of thread used in the criss crossing. I don't recall what I mounted it on - maybe coroplast like used for sign making or mat board. Something that wouldn't bend from the stretching of the fabric over it.