These clips break easily when removing the lower side panels on the Smart car. I designed these based on a Smart ForTwo 2013 but I expect they are the same on most models. I used PETG for strength and flexibilty. I didn't optimize the design for printing, I just modeled the same as the factory clip.

The H part slides onto the body panel with the circle funnel shape towards the car. There's an indent on the body panel that clicks the H clip in the right position.

Just press the body panel in place with these.