Good job, looks like you worked hard doing this. You could have made this job easier with a few changes. Strip the sheathing before putting the cable in the box, the wires should be about 2 or 3 inches longer and roll them into the box, it would be easier to make your receptacle connections. You could have used 6/2 Romex and a NEMA 6-50 receptacle, I doubt that the 240v compressor or the 240v welder uses a neutral wire. The white wire would be marked with red or black tape at each end indicating hot. Using 6/3 Romex is not a bad idea though, as you may need it later. Just leave the white (neutral) wire capped with a wire nut in the box. The other end to be landed on the neutral bus bar in the sub panel as you did. While using a separate grounding lug was fine, you could have looped the wire directly around the green grounding screw and then to the receptacle. It's only a 10 gauge wire, not a 6 like the conductors. The neutral wire which is wired in presently should carry no current, because a 240v circuit is a balanced load by default. The neutral connection on a NEMA 14-50 is for loads, e.g. electrical dryers that also have 120v circuitry for the low power needs such as in the control panel lighting, etc.
I used this lug. www.lowes.com/pd/Southwire-Lug-Copper-8-STR-14-SOL-2-PK/5001719777 I don't know if it's available at Home Depot, I got mine at Lowe's. Hope that helps.
