Took about 15mm of the top centre part tapered to the front and a further 35mm flat off the bottom
Also took some off the centre part of the backrest at about the shoulder blade height.
I used a fairly sharp bread knife to slice the foam.
I then used a pair of fence ring pliers and some hog rings to put it together, others have used zip ties with success.
I would recommend taking a bit off at a time, then trying the seat for a couple of days/weeks for fit before removing more.
Worked for me however others have not been as lucky
