From memory, if you cut out enough space to clear the piggy back bit, you'll need to raise the battery up a bit. You can do it with packing, or make a new lip for the battery to sit on. At the same time, the seat will need trimming.
Under the seat are all sorts of protruberances, including a big square one that sits over the battery. Cut it off. Don't worry about the strength of the seat, I'm, over 100kg's and never had a problem.
I did all mine with the bits still on the bike. Use a Dremmel.
Pete
