Since I obviously don't know how to stay within the parameters of my immediate equipment, the last platter I threw was about 21" across, and even without a Masters degree (sniffle-sniffle) I knew it would not fit well for trimming onto my 21" bat-never mind the drama of trying to flip the beast over on such a small confinement. And so yesterday I drove to my neighborhood lumber yard and bought a piece of plywood. For any of you interested in such a trip, be sure to realize firsthand that just because the inside of your vehicle measures more than 48", doesn't mean that the point of entry does. Fortunately, after attempting to help load the piece into my car with my friendly lumber guy without success (when he asked me what kind of plywood I wanted, I responded "the long flat kind" to which he was amused and not annoyed) he had no problem cutting down to 24" pieces, which is the size bat I wanted to make anyway.
So I made two of them with help from Adam's manly strong hands holding the wood for me as I jigsawed, and he ran the table saw, which I am quite afraid of, to rip the remaining wood down to made smaller bats. Another pearl of wisdom: the diameter is not the same as the diagonal. I know the old adage is to measure twice and cut once. I apparently prefer to measure five or six times, and then cut twice. There's a disconnect in my brain somewhere. At least my error was on the large side, so can be fixed.
This platter isn't actually finished yet. When I took it off and flipped it over there was still way too much clay on the sides so it went back on. But there it is, 21" platter on one of my 24" bats. It took every ounce of strength and commitment to flip this over onto the bat for trimming. Factor in 25 lbs. of clay, a 16" plaster bat, and a 24", 1/2" thick bat all sandwiched together, and being held up with the strength of my left arm. It's a scary moment.