For revision 1 of the rear wheel arches I have used some 34mm OD 2.5mm wall tube. I bought a hole saw to do the notching. Although Abracs was down the cheaper end of the hole cutters, it mentions it can cut stainless steel and has the pin locking system on the arbor so the hole saw saw is easy to remove. It all cam in at £10.40 and the arbor is the big one and goes up to 150 odd mm hole size. It cut the two notches I needed perfectly. And so with the help of a load of