I don't know of anyone that makes one.
The lightest plug that won't cause issues (like attracting moisture or causing corrosion) will likely be a piece of plastic rod or brass tubing.
Measure before you buy anything, but 0.375" (3/8") or 0.312" (5/16") diameter tubing or rod should fit inside the magazine spring.
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Don't even try it. It's even worse than the internet would lead you to believe.
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