I remember asking Gwyn about it when I came to fit mine - he said he wasn't sure how it was done, but said no one had come back to him about it so presumed it can't be too difficult to sort out.
This is how I did it - remove the guard leaving the filter and pipes connected:
Mark out the hole spacings on a bit of card, as you look at the chassis there are two top bolts that hold the filter and the bracket, and two bottom ones that just hold the bracket:
I decided that 4 bolts to mount it was overkill and so iirc elected to just use two ( a top and a bottom) and rotate the mount around the top left hole and fit it onto the top right hole on the chassis ..... if that makes sense.
So I rotated the bracket around the top right hole on my template:
and marked the bottom right hole by pushing through the template to mark the bracket:
I then drilled the hole:
But as I didn't have a large enough drill bit - I opened it up a bit with an old engine porting tool:
I then realised I'd done it on the wrong place and so repeated the procedure in the right place......
Then just bolted it up, the pipe work is stiff and there isn't much leeway, but if you do it like this it'll move enough to allow the new mounts to fit without having to remove or extend the pipework :
It immediately felt firm enough to satisfy me that 4 bolts was over kill. If it hadn't I'd have drilled a new hole in the chassis and used a nutsert. I've done lots of miles both on and off road on it though and its not moved at all.