I have never modified an ammo can, but have added handles to other things like the Vittles Vaults & gamma lid buckets. I found neoprene washers at Home Depot that work well. The plastic on the Vittles Vaults worried me because of it being fairly thin. I was able to find a small metal backing plate that was the right size (holes in the right place) to reinforce the inside. I think you'll be fine on an ammo can, just as long as you know that it won't be 100% water tight & bomber if you go upside down. Resist the urge to really reef on the straps securing it and you probably won't have problems with the loops pulling out or warping????
"We are all visitors to this time, this place. We are just passing through. Our purpose here is to observe, to learn, to grow, to love....and then we return home."
Australian Aboriginal Proverb