They should be the same except OTG has the hinged top grate. Get the one for OTS. Measure them. Any the same size will work. I have a top grate from an 18 on top of the charcoal grate and turned opposite so the small charcoal doesn't fall through the grate.
Smooth Smoke BBQ Team, KCBS CBJ, 2-WSM, Cajun Bandit Conversion, wsm 2880, Smokey Joe Mini wsm, Old Country offset, several kettles.