On my 18.5 I just drilled extra holes through the internal supports and outer shell midway between the top and bottom grates. I drilled 1/4" holes and installed short 1/4-20 bolts in the holes to support a third grate. It takes a little maneuvering to get the third grate past the top grate supports and on the new supports but once there it is stable. A standard 18.5 grate (like the top grate or a kettle grate) fits snugly so short bolts are all that is needed. IIRC the grate actually sits on the nuts so if you want to split the difference perfectly, allow for half the diameter of a nut when you locate the holes.
I plan the same mod for my 22.5 but hope to integrate the support bolts with handles to lift the mid section if the dimensions work out.