Here is a trick you can use to help hold a guitar pot in place when a lock washer can’t be fit in. You can bend the flat washer to give it a slight V shape. This V shape will exert pressure on the Hex nut and help the Pot from getting loose over time.
Quick review and thoughts on the Tapco Intrafuse AR-15 Spanner/stock wrench.
In short, it works as expected as long as you have mil-spec castle nut.
If you need help determining if you have a mil-spec or commercial spec receiver extension check out my video on the subject for a quick comparison -