I used a clay pot base in my old 18" bullet and it cracked when it was resting in the waterpan grooves.
Then I decided to ditch it in favor of a thrift store pizza pan foiled over and set in the waterpan. At first with red lava stones from and ancient gas grill.
Eventually ditched the rocks and settled on the pizza stone in the pan. I cook Dry - Hot n Fast.
It/they eventually crack over time as well. I sold all my 18" bullets and have two 22" bullets and an 18 & 22" Cajun Bandit Conversion.
I never bothered to look for any pizza stones or pot bases that big to fit the big bullet because the price wasn't worth it. What I did instead was have my welder friend laser cut some 1/8" steel plate discs that I use in place of the waterpan. $15 and will outlast the cookers!
I use the steel discs as heat diverters with a foiled over aluminum catering party tray as a drip pan. Cleanup is a breeze.
Here is one for my 18" Cajun Bandit
pizza pan drip tray which I foil over
You could drag your center section to a garden supply and try to find your size, possibly a ceramic/glass supply might have kiln platesetter, but unless you got deep pockets.....I'd go steel.
Just sayin'