The Rogue Fitness Loading Pin is a great tool for grip training, but the substantial weld on the bottom plate causes weight plates to sit crooked when in use.
This adapter will cover the welds and let your plates lie flat. It is 12mm (just shy of 1/2" thick) and weighs 56g (1/8lb if you're being accurate with your loading). It will clear a weld that is up to 8mm high and fits around a 1.9" shaft.
This will also prevent some of the metal-on-metal contact if you're using metal plates.