Flat outline for 3d object

Questions & Answers
1

I have googled for hours and still haven’t found a solution so I’ll start with this image:

My main question is:
how to make this yellow outline around the structure (in a way that doesn’t delete performance)? The yellow “lines” show up when mouse is over the structure, the game is 3d and the lines are as if they were part of the terrain.

2

Is there anything stopping you from using an unlit textured plane? If the camera angle is locked it should be fine being static. If not you can always adjust it to look at the camera.

Either way it would probably be the most simple way to deal with it.