This cap is made up of two parts the bottom and the top. If your 3D printer has two extruders you can feed the second extruder with a water-soluble filament like PVA to print support structure, so that you are able to produce the cap in one piece. The cap has a inner dimension of 78mm so it is made for posts with an edge length of 70mm but feel free to scale it up or down.
It is recommend to use a more durable filament like PETG that is my favourite. Normally I prefer PLA, it is easy to print, cheap and is not made for eternity. There is already enough plastics in the world. But depending of what is your application sometimes it is better to use more persistent materials.