Many thanks for your comments, it s always good to get positive feedback and I can see exactly what you mean! In the first photo I've posted you can clearly see a few things that will require changes (1) the angle at which the hose is at is too shallow - see the red lines (2) the height at which the hose comes out of the body is too low (this doesn't help with the issue you raised in your original comment), I can't actually easily measure this as I don't want to risk removing mine off the backpack and then not being able to put it back on, but I can clearly feel this. I would say it needs to be moved up on my model by ~5mm - see the yellow lines. (3) the feet position also needs some attention, in the photo you posted you can see that the exhaust body is much closer to the bottom edge of the seed tray and the front (uppermost) foot is mounted directly on the middle ridge of the see tray and the back foot is only just on the lower ridge (maybe by 1/3 or 1/4 of the ridge). I guess this measurement will vary depending on the seed trays you are able to obtain.
So, I will update my model; (1) increase the angle at which the hose exits the main body (2) move the hose up by ~5mm (3) modify the hose to compensate for the steeper angle (4) make the feet a separate part so these can be placed at the correct distances to take into account the seed tray ridge dimensions and (5) although you cant see this on any photos of the original backpack exhaust, the upper edge of the nozzle spout has a rounded profile as opposed to in the model which is simply cut flat (not quite sure how I'm going to do this with the modelling software I use but I will give it a go!), also its wall thickness is wrong, the original is only 2mm whereas my model is 3mm (I've no idea how I missed that!), see the second photo I've posted .