I made one from a small block of white nylon (maybe Delrin?)
To be clear, all I made was an insert that goes in a stock OEM magazine, the same as a bullet. The top is sloped slightly, and concave so the loaded round slides along it and is guided into the chamber.
The nylon follower gets pushed down into the magazine as the bolt closes, and pops back up when the bolt is opened, ready for the next round.
You still have to push the round into the chamber with the tip of your finger, but at least it lies where it's supposed to.