I felt like messing around in Blender and had some time, so I made an oversized bottom tab version - Revision 7 (R7). Those tabs are much wider, and a little bit thicker too. I’m on the fence about whether I should make the two side tabs any larger but that is also an option. They do need to be able to flex to snap together so they can’t be too solid like these bottom tabs.
I’ll upload the R7 version to Codeberg once I’ve done the steps below.
I also realized that I didn’t explain the post-modelling process so I’m going to add that in case others wanted to “fork” this design. And also so I don’t forget in the future. 
But if anyone does try it, be prepared to become expert in Blender Geometry Nodes.
During every step of the process you need to watch for things you’ve changed messing up the geometry and creating degenerate mesh stuff which a slicer won’t like. I use Blender’s 3D modeling plugin to check for issues (the main issue to watch out for is non-manifold edges on the Case mesh).
Starting point is the .blend file which does NOT have “Final” at the end of the name. I always increment the R revision number before making major changes.
- Make any changes needed to the case itself
- Make any changes (or fix an issues) to the modifiers, if needed. Select the checkbox next to “Case Split” Collection, to temporarily include those in the scene. Then deselect when done.
The bevel modifiers can be particularly problematic with major changes to the case because they rely on specific vertices inside Vertex Groups for the Case object (see the Case object’s Data tab). But also you need to check the tab connector cutouts on the top half, which are done via Geometry Nodes (Geometry Nodes or Geometry Nodes Focus work spaces). - Ensure that ONLY the Case Collection with the “Case” object is included in the view (It should be the only one with the checkbox checked in the Outliner.
- Export to STL format with the same name as the .blend file
- Open another instance of Blender, load the STL file, and delete the part of the model which is one part.
- Do another check for mesh issues with the Blender 3D Printing plugin with the two halves of the case selected. Some minor issues like Overhang Face and Non-Flat Faces should be acceptable. But definitely not Non Manifold Edges.
- Save this file with a new name adding “Final” to the end. Export the two halves as an STL, which should also be named the same but with “Final” at the end of the filename.
- Open the previous revision 3mf, immediately re-save it with a higher R number. Then delete the old model and re-import the new R version STL. Hopefully no mesh errors or warnings, and from here on out it’s just your standard 3D printing workflow.
- Inside the slicer, select the imported STL object and use “Split → To Objects”. Drag the top half of the phone case to the 2nd “smooth” plate (optional).