I've had the same trouble, and ended up just filling every other space in the ring. With a couple of rings, I'd say you could do 20 ears in pretty short order. Shucking the corn first will of course shorten the cooking time (and might arguably improve the flavor).
The roti ring or skewer ring would definitely work, though IMO the beauty of the corn-n-tater ring is the way it holds the corn right under the lid, allowing direct and reflected heat to magically cook every kernel all at the same time.