Found this on the floor of my 2017 Octavia Estate a while back. Googled the number but not able to locate it. Its wider than a seatbelt and quite heavy plastic.

It clips into the top of the plastic spare wheel retaining screw to stop it unscrewing itself. 

Well played that man: Spare Wheel Attachment Screw Škoda 5G0803899 (skoda-parts.com)

I’ll have a look in the boot later, I only recently got a full sized spare instead of the inflation kit so that must be part of it.

 

many thanks for the super fast replies