A strap that buckles in front is one solution. I went a different route.
I have 4 open points along the hem of my pack cover. At the "top," one opening allows access to the ends and to a spring-loaded barrel-type adjustable closure. Opposite this access point is one to which I added a "mitten hook." Two more openings are fairly evenly-spaced along the sides, one has a loop tied in its section of shock cord and the other has a second mitten hook tied into its section of shock cord. I can pull the elastic cord across the back of the pack and clip from either or both directions, depending on how full my pack is at the time. If necessary, I can then also pull the elastic tighter, and hold it at that tension with the barrel closure.