Um, what in the hell are you putting in a pot that large?
That's about 500 lbs of soil, probably more. You sure what you're potting requires such a large container?
FWIW, The pot under the huge Chinese elm below is about those same dimensions, possibly a bit larger. The tree is a century-old with a massive 20" or so trunk diameter. It requires a forklift and a crew to move it and to repot. It can't be moved for overwintering so a shelter is built around it.
The pot it is in now is a newish Chinese import. The pot was located through some direct importers a while ago. It took some looking around to find. I doubt very much a hobby, or even a professional, bonsai potter has a kiln large enough for such a container.
The only bonsai potter that comes to mind that has this kind of thing under their belt is Jim Barrett. He built the final pot for Dan Robinson's massive c
ollected Ponderosa pine at the National Bonsai and Penjing Museum...Barrett made the ceramic pot for it, but if I remember correctly, he had it fired in an industrial kiln...but that was years ago.