Seeking help with securing a strongman sandbag closed

Hi Knotters... I've got a sack of sand, 150+ pounds that I drop repeatedly... wondering what sort of knots you advise trying to keep the darn thing closed and tight.

Currently, the bag (Ironmind large) has a built-in bit of webbing and a buckle, but it doesn't take more than about 5 drops to go from being a compact bag of sand, to a loose floppy mess.

Seeking help from you who are dedicated to securing things with rope and knots, figure you'd have some ideas.