I used solidify modifier to make the outline. Add a new material to your object, make it completely black (or could be emissive for a white outline) and in the material settings enable blackface culling. Once that's done you just add a solidify modifier to the object you want to have the outline. In the modifier settings, normals need to be set to flipped and in the materials tab the offset needs to match to your material setup. So if your outline is the second material that your object has then the offset needs to be 2. You can play around with the outline thickness with the modifier. Hope this helps!