Model Rocket Motor Plugs

To check a model rocket for stability one of the easiest things to do is a swing test. To do this you put a motor in the rocket, tie a string at the point where it balances level, and swing it around (the string should be 10' or so long for this). If the rocket flies straight, it's stable. Or so the theory goes.

But I don't always have motors available for this so I did some research and some math and determined that an Estes D sized motor is almost the same weight as 18 pennies, as long a