If you are running a flip flop hub with only one cog or freewheel here's a great little cover for the thread on the other side. It keeps the threads clean and undamaged and weighs almost nothing (okay, 12g if you must know).

It comes in a range of colours and includes a spacer if you are fitting on freewheel threads which are a little deeper (at least on the Sturmey Archer hub that I tested one on).

Standard 1.37" x 24TPI threads.