Well I really liked that commercially available gizmo that you screw onto a soda bottle and inject your brew with gas from your CO2 tank.
Unfortunately, I kept misplacing them. At eleven dollars each it gets expensive fast.
Hmmm, automobile valve stems, that's the ticket. About a dollar a piece.
Valve stems come in different sizes, both in length and diameter. The diameter is specified for the size of the hole in the tire rim. All you need to do is drill that size hole in the soda bottle cap. Now the tricky part. You need to pull the stem through the hole. You can't push it through! If you try pushing on it, it gets fatter. Pull it, it gets thinner.
To make things easier, get one of those combination tools tools for replacing the valve stem valve and for cleaning the threads. Then slip the stem through the hole, screw on the tool and pull. For another dollar you can buy a metal valve stem that is threaded its entire length. You simply insert it through the cap, place the washer on top and tighten down the nut.
To inject CO2 I connected a male ball lock connector to an air chuck. Now I can quickly remove the gas line from my beer keg and give the soda bottle a quick blast. If you don't have an extra male ball lock you can use a hose barb and air chuck connected to your CO2 gas line.