Miniature landscape with dynamic loading only specifc parts

What would be the best way to create a miniature landscape with meshes and foliage (like a diorama)?
How can I dynamically only load a specific part?
E.g.: I have a sphere/box and the part of the landscape that is inside the sphere/box should be the only part that gets loaded and rendered.
I’ve tried using a “custom stencil buffer” and object masking with a BoxMask but that was not the result I was looking for.
Any ideas are helpful.