This little cassette spacer is the answer for the question of what do I need to run a 10 speed cassette on an 11 speed freehub body. Simply insert this onto the free hub body, then the 10 speed cassette, tighten down the lock nut, and you're good to go.

The spacer is 1.85 mm thick and has three notches to keep it from spinning about.

Compatible with Shimano and SRAM freehub bodies.