They can be a real pain in the arse to fit. The moulds used are probably pretty lame to start with but as the covers are made out of ABS they tend to 'relax' after they've been moulded to shape.
The only thing to do is a trial fit to see where they need tweaking, once you've sussed that, plunge the buggers into a bucket of very hot water, after a 30secs or so take them out and immediately bend them to the shape you want. Hold them in this position until cool and then trial fit again. Repeat the process until you're happy. Probably best to wear gloves when you do this cos very hot water is err......very hot :lol:
Cheers
Ian