One, from David, based on authoritative experience.
Two, from a friend today -- simply combine the standard USGI utility strap:
with the standard USGI .30 caliber ammo can:
Clip one end of the strap to one of the handle attachment rings, then repeat on the other side of the can with the other strap clip.
Try it with a loaded ammo can, and you too will be a convert.
Here's another view of the utility strap.
Also, if I may remind readers of the Universal Load Carrying Sling from a Praxis post last December, Mission Loads -- Sustaining Light Infantry on "Shank's Mare."
You can carry two ammo cans with it, as well as other nifty stuff.