The floor and ceiling aren’t one piece. They’re made of four 30357: Black Plate, Round Corner 3 x 3 each with an assortment of other square/rectangular plates to fill in the gaps.
It looks like there are 20 studs between the corner pieces on each side. The second image you posted shows a layer of tiles holding the floor together. Presumably there is another layer of offset plates holding the ceiling together from the bottom.
Which pieces you select for the center of the floor/ceiling will come down to two factors:
- Price, or how expensive they are to get (assuming you don't already have them).
- Strength, or how easy it is to make a sturdy structure (assuming you aren't planning to glue them together).
Here's one possible tiling that favors strength:
And a layer of plates you could place on the underside to hold everything together:
In these examples, every joint on the first layer is overlapped by at least two studs on both sides by the second layer.
Here's the parts list for this tiling, along with the cheapest price on bricklink if you're buying each part from one buyer:
Remember to double this if you're building both the floor and ceiling with the same layout.