First off, Drew P.
I've been cooking on a Primo Oval XL for about 11 years now. The ceramic plate at the bottom is there only to prevent the bottom of the shell from coming in contact with red hot charcoal that may get through the bottom CI grate. When in place, the plate leaves about 3/4" between it and the firebox and exposes the same amount of the bottom shell. Most particles that fall through the CI grate are small and mostly ash, so they are not as hot as the center of the fire.
On at least one occasion, after a shop vac cleaning, I forgot to replace the bottom ceramic plate and did a 250* 5 hour rib cook, after which I let the temp rise to about 550 for 20 minutes to burn the dome and grates clean. There was no cracking or visible damage to the bottom shell. I haven't noticed any latent damage over time.
So, I'd say get a KiskAsh (or generic) charcoal basket, keep the CI grate in place, and go ahead and cook. I would go ahead and order a replacement ceramic bottom plate or buy some kiln pieces that you can make fit in the bottom as best as you can, or even put a 1/2" layer of sand in the bottom (that you can vacuum out later) and avoid really high temp cooks until the new ceramic bottom is in place.