Why make two separate rings on the splash bowl? Why not just a small piece to cover the slot for the girdle? It can be a simple piece that hangs over the edge like on a Facette, or since you’re designing it from the ground up it could slide into or around a notch you design on the edges girdle slot.
I’m wary of 3D printing splash bowls. 3D printed parts can leak if exposed to standing water, like if the drain clogs. Ruining a motor or motor controller isn’t worth the risk unless there’s no other option. If you make one for someone else, who’s responsible if it leaks? I would suggest sealing the bowl with a sealant to be safe.