1/2" diameter PVC plastic pipe.
It is our good fortune that very cheap and easy to find 1/2" PVC pipe fits snuggly inside 2-liter bottles. When used to connect two bottles, the pipe keeps them from flopping around.
Some hardware stores might sell it by the foot (you only need 2 inches per project). From other places, like building centers, you will have to buy 5 or 10 feet at a time. It is still little more than a dollar for 10 feet. PVC pipe is white. Do not get "CPVC" pipe, which is more expensive and a tanish color. One more confusing thing: 1/2" diameter pipe is actually bigger than 1/2". Go by what it says on the pipe, not by actually measuring the diameter.