I think I got mine for OSH as it was the cheapest locally, and slightly cheaper than Amazon; $7.99 if I recall (it wasn't that long ago).
In total I'm using an 18 1/2" Kettle Charcoal grill on the bottom, with the bottom of the pot removed. As discussed earlier in this thread, that grate pushes past the ridge in the pot with a little force, and your diffuser (12" terracotta saucer in my case) or water pan can sit directly on this. Two Smokey Joe cooking grates then go inside to cook on, one resting on the ridge, and the other 3 3/4" from the top.
If you have difficulty pushing the charcoal grill past the ridge, you could rest it on the ridge, saucer on top and then cooking grate on top of that. This results in less usable cooking space, but it's still very workable - it's what I'd done up until yesterday!