Alright, heres my ghetto paint picture to give you an idea.
So Like I said, V cut the seat (red lines) and then flex the sides in, and rejoin the cut out section (blue lines) to make a skinnier seat, without losing your pretty sides. There is a few ways to join plastic, a soldering iron works.. Just go to a local do it all shop, and see what they have. Some products made to fix abs fenders for cars will work. Once modded, you can sand down the seams, and paint over it..making it barely noticeable.
I say all this assuming your seat flexes fairly well..And I have never done it, but its logical.