On my last outing I managed to lose a bottle of water purification pills out of my canteen pouch. That could have been a disaster since my water filter plugged up. Fortunately I had a second bottle so things worked out but I don't want that to happen again.
Since the pouch on a canteen cover is held in place with Velcro it's fairly easy for a snag to open it and once open you lose what's inside. Those of you with good hearing might hear the zzziip when it opens but I can't so gone is just gone. So that doesn't happen again I made a very simple keeper for the bottle. Now, even if the pouch opens I shouldn't loose the bottle.
The Potable Aqua bottle has a lip around the top so I tied a loop in a piece of cordage and slipped it around the bottle's neck.
I then snugged the loop down to firmly hold the bottle.
I attached the other end to the Alice clip on the canteen cover
And stuck the excess cordage into the pouch so it doesn't get snagged on something
There is enough length to the cordage to allow me access to the pills inside. This should work whether you use your canteen on a pistol belt or on your pack as I do.
I repeatedly dropped the open bottle to see if the cordage would prevent spilling the pills and had mixed results. I admit I was pretty rough in the test. On average I lost about 3 pills before the bottle righted itself at the end of the cord. If it just slipped out of your hands I think it would save you some lost pills as opposed to dropping on the ground.