The top has to be lifted straight up at the front and back at the same time.

I found Heavy-Duty Suction Cups (Pair) to work best, but due to the structure of the plastic they quickly loose their grip, so work fast.

Alternative: Lift the top cover in the back as little as possible and use Plastic Cards to work your way to the front until you are able to get a grip of the whole top cover and lift it straight up.