Dont bother cutting it !!!!
Get your self some MDF and use your old one as a template and make your own one from scratch mate, im no DIYer but i made mine myself and it works great.
You can get the special carpet (acoustic carpet??) type stuff to cover your shelf with from Halfords as well, that way if you ever want to sell your polo you can keep the new shelf and your speakers.
it will look a lot tidyer too mate, the chavs wont be able to see your speakers too, which is an obvious bonus. That way you car appears to be standard

i think it looks neater as well.
Remember all it takes is a half pissed up chav to see your speakers in the rear shelf and smash the window and run away with them, so not worth it geez.
Calâ„¢