Supposing you have already created the form of your seat as a single skin:
Then one way to retopologize a portion of it, so you can simply remove faces to create the slots, would be, with a helping hand from Geometry Nodes, to extract 2 matched edges from your form:
The top edge is duplicated from part of the boundary, the bottom edge is the result of a horizontal Bisect operation, with both the 'Inner' and 'Outer' cleared. They're converted to curves, which can be resampled to matching counts with a tiny GN modifier:
So, as on the right above, they can be Converted back to Mesh, linked with Bridge Edge Loops, and have a couple of even cross-loops cut in.
The cross-loops can be bevelled with 2 segments, and a selection made as shown above, with the help of CtrlShiftNumpad +. If you delete those faces, the resulting topology survives Solidify, Bevel , and Subdiv modifiers, when eventually assigned.
There various routes to deriving the seat-base from this geometry (Grid-Fill and projected Shrinkwrap to your original form?) .. I guess that's another question.