Following the directions in this Instructable, I combined the folding knife side plate with the side of the iPhone case.
Once you have your .OBJ files, I followed ShadyLogic's steps on how to edit with MeshMixer and was able to quickly import 2 models. Import one model, then import another making sure to "append" the project to have both models in the same environment.
Using the alt+right-click/alt+left-click mouse button(s) I was able to navigate around the MeshMixer environment and line up my models. I placed the inside edge of the folding knife and lined it up with the inside of the iPhone case. Pay attention to where the knife siding penetrates the plane of the inside of the iPhone case.
Merge the two models by selecting one model then Shift+Click the other model (they should both turn white). Then go to Edits > Combine to merge the models into one.
Now we can export the merged model back to NetFabb so we can make it watertight before sending it to print. In Netfabb go to Extras > Repair then find Automatic Repair on the bottom right, then execute the repair. You now have a watertight, custom 3D model to send to the printers. Save the model as an .STL or .OBJ (depending on what the printing service requires).
After you've got your .STL or .OBJ file you'll need somewhere to print it. There's companies like shapeways and i.materialize, the pricing will depend on the volume of your model.
My iStab was printed on the Objet Connex 500.
I included the .STL file you'll need to print your own iStab here ↓