My Fabia 1.4 plastic engine cover is rattling/buzzing in the area of the oil filler.

Is there a simple cure like jamming a piece of foam between cover and engine? Problem is it

The cover doesnt go down near the oil filter?

The cover has three mounting point. One 10mm under the cap as the back. One 10mm under the badge and a rubber push in mounting at the back on the left hand side. With all these in place it should be solid and secure.

On mine the rubber push in mounting was loose against the 'cover side', and it rattled rather annoyingly. One day I will glue it down, so far I've just given it a good whack and it's now stopped vibrating.

Bit different on the MPi, the mounting is part of the cover.

The 1.4 Mpi engine has the 3 mountings Lummox mentions plus there is a rubber seal around a cover part which slides into the oil filler on the engine. This seal was worn and cracked allowing the cover to vibrate up and down. A new seal stops the vibration.

There is a spare oil filler blanking cap mounted under the cover. Presumably you use this to blank the filler when the cover is off to stop dropping bits into engine. You can take the seal off this as it’s probably unused and so in good condition. But don’t put the old seal on this blanking cap as it will fall off and drop onto road without a good seal to hold it in place. Use a small BLUNT screwdriver prised under the seal like a tyre lever to get the seal off.