Hi,
thanks for asking. Write up below assumes your starting with very limited knowledge I have, so apologies if you know all of this, thought I'd write it up for anyone to follow.
Uniwersal [sic] Honeycomb Black Plastic vent car tuning Grill Mesh 40x100 cm https://www.amazon.co.uk/dp/B01M035FL1
Cut a strip that was exactly 5.5cm high from the bottom edge using a Dremel or very fine saw. (Gave me excuse to buy a Dremel for about £50). Bottom edge of the sheet is already smooth, top edge was inline with one particular straight edge of honeycomb which made it easy to guide the cutting wheel along, though important to go slowly as one wrong cut and you'd have to start again. I can't remember the width, I measured along the open edge of the grill with a piece of string and left some extra - and then trimmed it afterwards to fit.
You'll find that the bottom section of the grill has a ridge which will retain the bottom edge of the panel you make, and that at 5.5cm the top edge will side just high enough inside the gap to push against the back of the open section. Once inside, it was just a matter of getting cable ties into the right places at the very top and bottom of each vertical support on the grill, and fastening them tight inside the space behind the grill and snipping the ends of the tie (leave a tail, in case of the need to tighten further). Be very careful to make sure what you snip!
Getting access to fit the grill was quite simple. Remove the bonnet opening lever by depressing the small rectangular indent in the top. I put something big and soft under each bonnet strut so it would stop the bonnet closing if I tried to close it without fitting this lever back again!
Undo the 5 x TORX30 screws and washers holding the plastic slam panel in place, and then pull it gently upwards at the bottom side where it clips into the top of the grill. It pulls off quite easily. Then also remove the 2 x TORX 30 screws holding on the plastic air scoop on the left hand side. This gives you full access to the space between the radiators and the grill. Depending on spec there will be various cables to be careful of - the park distance sensors, the radar for the adaptive cruise control, the ambient temperature sensor, etc. Any of that lot getting unplugged or disturbed is a trip to the dealership.
For the first go, with my slightly longer piece of trim, I had a spare bit left over which is how I worked out that the 6cm I thought I'd measured was better at 5.5cm. At the passenger side there was a cable at the very end of that bottom grill section which I had to be careful of, so slide the panel in between the cable and the inside of the grill section. There is a cable running all the way along the top of that lowest section which is why I couldn't cut a large piece and fit it flush behind the whole lot. However, I think this gave a better fit. You could do the upper section between PDC sensors and the number plate section of the grill, with another section of the mesh, but I chose not to at this stage.
Once the section was the right length, with about half a cm on either side to spare, it is just a case of cable tying on to the upright pieces in that section of the grill. Get someone to help spot when you're poking the cable tie out from inside so that you get exactly to the edge of the upright, then poke back in exactly on the other side of the upright but make sure you also go into another segment of the honeycomb on the way back in. Then, carefully reach both arms in to the space to pull tight and snip off the ends of the cable ties.
The edges of the new panel are not secured, so I need to see if a motorway journey results in them being pushed backwards at all. That could be a problem so if it does happen, I might have to find a solution for it in case it fouls any cables. At the moment though hoping not to. Will test tomorrow!