OK - you create two slots in the back of your pannier/top box each the width of the strap by the equivalent thickness of the strap so that the "lugs" or ends of the strap go through the skin of the pannier. A small pin is then dropped though a hole in the top lug to hold it in place. The bottom lug has a 90 degree bend in it that holds it in place
No need for padlocks etc as the pannier has to opened to withdraw the pin
To save making a groove in the bottle a horizontal strap attached to the middle of the vertical one stops the bottle being extracted sideways
A five sided box can be similarly attached to the pannier to hold other accoutrements
When the bottle/box is not required small adhesive (Tosser?) stickers over the slots keep the pannier watertight
No need for padlocks etc as the pannier has to opened to withdraw the pin
To save making a groove in the bottle a horizontal strap attached to the middle of the vertical one stops the bottle being extracted sideways
A five sided box can be similarly attached to the pannier to hold other accoutrements
When the bottle/box is not required small adhesive (Tosser?) stickers over the slots keep the pannier watertight
oobottle printed onto retaining strap should sort all but the most desperate from stealing it