Tailcap o-ring for Ultrafire HD2010

Hi: Just got to looking at my Ultrafire HD2010 and noticed there is no tailcap o-ring??……anyone know the proper size?(linky please?) Thanks,HB

The stock o’ring on mine measures 22mm ID x 1.35mm in section. I tried a 24mm ID x 1.5mm section o’ring which is a better fit.

Thanks a lot! HB