It has gotten pretty quite on the X-Type, S-Type and X308 channel: No one is asking anything. Thus I figured, I post a brief DIY Guide:

Actually, I was just debating with my wife, if I should write this post here, or keep it secret and start producing them for profit - but we came to the same conclusion as always:
Every idea, every invention, every product idea is dead in its tracks in Australia due to the horrifyingly high postage costs in Australia. (Background: The Australia-Post-CEO received last year a remuneration of 2.7 Million AU$ for his incompetence of making a loss of 88.5 Million AU$ in that same year DESPITE those horrendous postage costs.) Thus, as Australia does make sure that way that all new ideas are dead in their tracks, I could just as well post my idea here:

I have a number of jerry cans (petrol cans), but I lost all the screw-on filler nozzles over time, and even if I still had them: Those nozzles were most of the time shaped like the nozzle of the blue funnel shown on the picture below. This is in my eyes a very faulty design to start with, as quite some petrol will remain in those useless grooves and it will either spill or evaporate. But worse than that is: If I would intend use the blue funnel to fill a jerry can into an S-Type (or most other cars), I would have to grow a third hand first, as I need 2 hands to hold the heavy jerry can (5L, 10L or 20L) and a third hand to hold the funnel in place. Failing to hold the funnel will cause the filled funnel to flip over while filling and cause a huge mess.

Thus, as the blue funnel from the shops is a No-Go, I bought a PVC-pipe, which matches the lower end of the green funnel, which I bought (which however could not have been used as is to fill petrol into any car, as it does not open that little "door" behind the fuel-tank-cap, and it would also require a third hand.)

I have cut a length (see picture) off of that white poly-pipe. I bend it (using a heat gun) and attached it to the green funnel with a rather big bit of heat-shrink, which comes with glue inside. I then connected the poly-pipe with the funnel using the heat-shrink and the heat gun again. As I result I have a sturdy and solid funnel, which sits in place, no third hand required, it pushes the little flap (=door behind the filler-cap) open and the funnel does not flip over, when pouring petrol in there... Another huge advantage is that the poly pipe I chose has an INNER diameter of 17mm, which is more than on all the funnels I bought have and as on normal fuel-filler nozzles, which means, filling the petrol into the car is way faster and you don't have to lift the jerry can that long...

And note how I fixated the filler cap on the fuel-cap-latch-door: That is design-intend, but very few people know about it...


PS: The OUTER diameter of my poly-pipe is 21mm. And this is the maximum you can go for, because other wise it will not fit thru the "flap-door" behind the filler cap anymore. And if you go for less than 21mm, the funnel will probably not be held so nicely in place, and you would need that third hand again to stop it from flipping over.

 

Very nice!

A couple of quick thoughts:

Is the plastic pipe fully resistant to gasoline? You mentioned it was PVC and that supposedly is. However, I was wondering if what you have is true PVC or perhaps some generic plastic lumped into a common category. I'd hate to see it soften and fail at some critical time.

Have you given much thought to version 2.0? What about modifying a spare fuel cap? Drill a large hole through the middle to hold the funnel even more securely. It doesn't have to seal, but merely act as a guide.

 

Good point, Karl.
I double checked - the length of poly pipe, which I bought in a hardware shop, is labelled PVC-U DN15 PN18.
I asked google about it being petrol resistant right now - the answer was kind of yes and no. It does not seem to be petrol resistant above 50°C Celsius (122°F).
So yes, you are right and at high temperatures petrol could soften the material, but considering that I will not reach those 50°C and that the material is not in permanent contact with petrol but only for very brief moments in time, I am happy with my own DIY funnel as is, however, for anyone wanting to copy it, you might want to search for a more suitable material.

Btw.: That "PVC-U DN15 PN18" also tells me that the OD (outer diameter) is 21.2mm. That is exactly what fits thru the "flap" behind the filler-cap.

PS:
Fun Fact: I know quite well that some plastics do not appreciate contact with petrol... When I worked as Engineer in Germany (I better not share here for which company and which car-model for confidentiality reasons) my job that in that instance was putting together issues during the development phase. One of those tests was that the new model is being driven round and round in a big circle on a test track 24/7 to get as many kilometers as fast as possible. And a mystery issue was that there appeared suddenly cracks on one of the rear lights... The reason: During their hasty refuelling stops there was apparently an occasional spillage followed by driving the car again full speed so that the remaining drops of petrol near the fuel filler were moved by the wind (airstream) onto the rear lights, which did not like the petrol and cracked...

 

You raise a good point. PVC is generally considered resistant to gasoline, but not all plastics sold under the “PVC” label perform the same. Some blends can contain fillers or plasticizers that break down faster in contact with fuel, which could lead to softening, swelling, or stress cracking over time. If it’s true rigid PVC, it should hold up, but it’s worth periodically inspecting the piece to be safe.

I like your version 2.0 suggestion with the spare fuel cap. Using it as a guide rather than a seal would add stability, reduce wobble, and keep the funnel aligned without much extra complexity. Seems like a straightforward upgrade.