Whilst rummaging around on the internet, I came across a ready-made air duct assembly from Europa Spares - https://www.europaspares.com/AIR_FILTERS_and_INTAKES/ - which at £37.54 delivered looked as if it would be very useful. After a good look under the bonnet, I reckoned I would be able to connect to the air filter housing elbow and run the duct through to the side inlet.
Unclip the back of the hood and remove the shelf liner and engine cover (you probably know how!)
Take the top off the air filter box, and remove the filter. (I've not gone for anything other than the standard filter as yet - there could be further small improvements from other types, but mine is still quite new and unsullied!)
Remove the filter box - there's one bolt at the forward end to undo, and a plastic locking pin to pull out at the rear, and it can then be manoeuvred out.
Prise the elbow out of the resonator box that sits below the filter box, and remove it along with the short connecting duct.
At this point, I replaced the filter box to get some more temperature measurements. I relocated the sensor to the bottom of the filter box, mounting it on a piece of foam to isolate from the box itself.
With the air intake now directly into the box, the temperatures went sky-high, easily reaching 50degC on an averagely mild day. I then attached a short length of plastic rainwater pipe to the elbow, so that the engine would draw air from nearer the side vent. This dropped the temperatures quite noticeably, but still only to about what I'd measured before. My fingers were crossed that taking the duct through to the side vent would take the final step.
By now you should have purchased the duct assembly - I went for the black one, and hoped that the 63mm diameter would fit the elbow OK. The kit includes an attaching clip, an intake gauze which can be omitted, and a couple of heavy-duty tie-wraps which go in your bits box for a future job. A quick check against the redundant elbow showed it to be a perfect fit. The duct comes in a 1 metre length and has to be cut to the right length. I found that shortening it by 33cm allowed it to fit neatly.
Remove the two screws holding the body side vent outer frame, then the three screws holding the inner frame. The flexible duct will go through the inner shaped duct with a bit of persuasion. Feed it round the corner and up towards the air filter box. The picture below shows the duct before it gets trimmed.
Push and pull the duct until it is as far into the side vent as it will go.
It will sit quite neatly in the inner shaped duct flange.