help

Can you show an image of what's happening and your S4S file? There's a lot of reasons why a Sim might be misaligned on a couch. From what I can tell, misalignments usually happen when the couch is weighted incorrectly (resulting in the game's calculations for movements that interact with the object being off), the slots are not bounded to the correct rigs, or the rigs have not been reset using the S4S Blender widget after making changes (which is necessary to update the game on what rig configuration the object is supposed to load with).

The good news is that you probably don't need to make a new rig. Since the item has EA objects that function equivalently, you can simply replace an EA object's mesh with your own and use that object's rig, slots, and weights. S4S has a tutorial on how to do that here: sims4studio.com/thread/826/create-object-mesh-package-existing?page=1

Also, under the heading "Object Rig, Slot, Light, Model, and Footprint Tutorials," there's a list on the S4S forum of links to tutorials that talk about rigging and slots in more detail: sims4studio.com/thread/645/index-custom-content-tutorials-build The whole post is extremely helpful, actually.

And there's Youtube videos on custom functional furniture, such as (but certainly not limited to) this video by Summerrplays or this video by Rain. They don't only cover sofas, but often include them as well as information that is applicable to them.