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.
Plastic coat hanger maybe, but it sounds like something that might bow to the side a bit too easily.
Why not go to the local home improvement or hobby store and pick up a wooden dowel?
No plug will be necessary for pistol caliber rifle in Ohio. When addressing that issue, they realized that it would not be practical. You will have a 3 round limit you can put in the gun, but the magazine does not need to be plugged.