How to create rectangular-curved extrusion into curved wall?

Hi, I am trying to make curved openings, following a rectangle which I pulled onto a curved wall. I want to create shelves into this wall, which are extrusions into the wall. However, I’ve tried ExtrudeCrv and it warps the edges, and I’ve tried offsetting and creating a polysurface and it also warps. Can anyone help me with this?

This is what I have:

This is kind of what I am trying to create for the shelves:

simply extrude the entire bottome edge upwards by the material thickness. then copy it upwards and make the walls the same.

if its just for a design proposal and the curves you show here are healthy you can use these to split the round surface then use OffsetSrf from the resulting surface.

there are many fast methods. if nothing works you might have issues with your curves through pulling or whatever you did. always better to post your geometry and not just some images.

Many ways to go about this. Here is one method: