This plug fits into the little nylon pockets that hold the ends of the tent poles in many children's play tents. The poles tend to poke through the pockets almost immediately, causing the tent to become unstable.
I found the easiest way to use these to insert them into the pockets, then put the tent poles into the hole.
The FreeCad file is included in case you need to make adjustments.
The author hasn't provided the model origin yet.