Apply some silicone lubricant (eg furniture polish) onto a small cotton wool bud and wipe it around the seams of the plastic case. Wipe it around where you expect the case to slide. Take care to put the lubricant into the joint rather than the outside of the case. Leave the case upside down for a few hours to allow the lubricant to penetrate the joints. While it is upside down, check for security screws and tabs.

Turn it back up again. Place it in the sun or next to a moderate heat source (max 30 C air temp) to warm the plastic. Flex the case by pressing the top and bottom -- say 25mm in from the sides. Use your fingers until you see deformation on the top cover. Good luck.