What is this part - air intake for Skoda Octavia

Judging by the similarity to my Octavia's intake system (which is also a TSI engine) it is a simple cover that redirects intake air flow from the open parts around the bottom of the housing back down through the round opening in the centre and though the pipe into the the air cleaner. Mine comes via a pipe off of the the left hand side of the missing cover and turns back toward the air cleaner, but judging by the picture yours just goes down that hole and to the air cleaner.

The Skoda workshop material I have refers to that housing as a 'connection fitting', the cover that you are missing as simply the 'cover for connection fitting', and the hose from the fitting to the air cleaner as the 'connection hose'.

The front of the fitting is a vent (basically just an open hole through the front panel) to feed cold air from the outside, and to the sides and rear it is pretty much open to the rest of the engine bay.

I guess the design allows for the cold air to be siphoned off as required (or allowed to pass through to the engine bay) more smoothly as it goes through the fitting rather than being rammed straight into the intake system.

Without the cover you're going to be missing a lot of the the cold air from the vent and sucking more hot engine bay air into the engine from above.