Yes, the bottles work. I bought 3 dozen mallard years ago. Repainted one dozen into blue-collar. Filled the hollow keeps with sand. Painted the bottles and applied the detail decals on the mallards. I killed several ducks over them.
The downside, the plastic bases that used be on the bottom of the 2 liters are required for bottles to be held in place in the decoy frame. Often the bases would split or come unglued from the bottle.
I'll dig some out of the barn and share some pics.