waterproof mega 2560 enclosure with room for 9volt battery?

i cant find a waterproof mega 2560 enclosure with room for 9volt battery

Do you mean a PP3 type of 9v battery ? That is unlikely to be suitable to power an Arduino for more than a few minutes.

...R

Check for IP-rating when searching for enclosures, it describes how good enclosures must be protected to different environments.

Depending on how waterproof your project needs to be, you could choose an IP64 (some) to IP69 (best) - rated enclosure.

You could try polycase.com, pelican-case.com, otterbox.com Mak sure it's large enough to isolate the battery from shorting things on the board.