If I need a plug, I usually grab a pencil. Works for me.
I don't recommend what I saw a few years ago. The warden was making his rounds around the dove fields and several of the guys grabbed green twigs and shoved them in. A year later, after they forgot about the twigs, they had to replace the magazine springs and polish the inside of the magazine tube. Hunks of rust!!
A wood pencil is a good substitite for a plug. I also have a dozen or so plastic plugs that require me to take out the retainer. A dowel or pencil goes in without having to fool with the retainer and the strong magazine spring. Just measure the pencil and drop it in. No moisture to ruin your gun.