Here's a trick I used to keep the cover snug on my big board. I use elastic in a casing around the edge and then use lengths of elastic with a "mitten" clip stitched on each end. I just clip the elastic on the saggy area and bring the other clip to the other side and clip again.
I usually cover with duck cloth but love the idea of ticking. Next re-vamp, ticking! Thanks for the idea.