update:
I decided to stop fannying about, and I got some 100mm black hose/duct from Bill - and its good stuff. Expensive for what it is, but it'll be nice and strong, and do the job nicely.
I've also gone and smoothed my airbox - i'll do some pics at some point. Just took the main ribbed bits out from inside.
I've trial fitted the 100mm hose into the std airbox hole, and it fits with zero modification to the airbox hole. I cut along the length of the hose, about 4 inches long, folder the hose overitself a bit, making the hose diameter smaller at one end. It then slotted into the hole quite nicely.
I would assume this is better than just smaller hose, as at the other end its obviously got a bigger opening, meaning more air will be caught. Once in the hose -with nowhere to escape - it HAS to go into the airbox.
I've got some battery relocation brackets on the way, but i don't think i'll need them if DPJ is right - i could fit the hose down the side without.
Hopefully gonna fit this weekend - and then try and source a green filter from somewhere -and thats a 30second swap.