A customer contacted me and asked for a part that could fit in the end of a metal beam he had and hide what was inside while allowing the air to circulate. I made this insert that is printed in two parts and assembled with 3mm cap screws.
If you print it and it is too loose for your taste, put a rubber band around it and it will be stiff enough to stay in and still removable.
The author hasn't provided the model origin yet.