I have one of the tru remotes and if you were to buy one, make sure you get the 12 volt version. The first one I got was a 120 volt and it plugs in like a regular remote, but the only problem was that your machine will stay in high idle mode and I didn't like that so I exchanged it for a 12 volt system.
You just connect the two wires straight to the welder battery and the other cord plugs into your remote outlet, it works awesome!
They do make them for the 300D and most of the other Lincolns and Millers.