The retention of the lid can be improved by using 'U' nuts on the flange for the self-tapping screws to tighten into. Use RTR5032 nut and SS1060462 screw.

Sealant may not really worth it as the small amount of  unfiltered air entering will not affect the engine, however you can make an instant gasket with a tube of RTV rubber sealant. This can be purchased on the internet. Farnell sell a suitable product.

To ensure the sealant sticks to one side only, say the main part of the box, put cling film on the mating face of the lid and screw the lid down. Then the sealant has set you should have a  suitable gasket that will prevent almost all air entering and should prevent rattling too.