This is a rotatable cup which can be used to catch rolled balls, toy cars etc. designed to work with my windw- stick ping-pong ball track pieces:
https://makerworld.com/en/models/372642
 

It has an M5 (5mm) metric fixing hole which can be used to mount suction cups for windows, bolts, screws etc for fixing. I've used suction cups which neatly attach to windows, mirrors, tiles, whiteboards and bath porcelain.

I've printed a ton of these as they are a huge hit with my toddlers.

Link to a solid 3d printable pingpong ball, perfect for this track:

https://makerworld.com/en/models/372740

Link to the straight track component shown in the pictures:

https://makerworld.com/en/models/372642

Link to the M5 bolt suction cup pieces used in the pictures:

https://www.amazon.co.uk/dp/B0BTD3PQRK