Ensure that the clear silicon seal is in good condition, is lying flat, and is not fouled by any plastic debris or other material. Close the housing, and gently squeeze it shut. Foam on the lid will compress against the batteries, holding them firmly in place. Tighten the 8 screws to uniform tightness - such that there is an even gap at all points along the interface between the base and lid.
The screw heads are Posi-Pan head. As such a Posidriv bit should be used. A Phillips screwdriver can strip the threads. The screws are a thermoplastic screw: 2.5mm x 8mm (BN82428). If you are after a stainless steel variant, the product code to source is BN82429. It is a good idea to use a screwdriver with a torque limiting clutch, start with the screwdriver on the lowest setting, and gradually increase the torque until there is a small gap between the base and lid (no need to overtighten as you risk cracking the housing) The final torque when finished will be around 0.7Nm most of the time. (Value is a guide only)