This is a model of the light grey plastic plugs that are used in the sides of the bed on the Corvair-based rear engine pickup trucks, the Rampside and Loadside, produced from 1961 to 1965. These parts are no longer available.

Since they are located outside, they should be printed in PETG. It is possible to print top-down but I had the best luck printing top-up with supports under the insides and edges at 240 / 60 degrees C. Use .15mm resolution with 4 top layers for best results on the thin edges. 50% infill for strength.