The standard for highly protective, waterproof cases could well be Pelican cases https://www.rei.com/product/504064/pelican-1400-pelicase . These are available in even larger sizes. They are not at all cheap, but are commonly used to house relatively fragile instrumentation of all sorts.

Somewhat cheaper would be dry bags,with a closure that folds over several times and is secured with a plastic buckle. There are several manufacturers of this basic bag, and they are available in many sizes (REI has a good selection, among many retailers). Properly secured, these have kept things dry for me on boat and kayak decks, with no failures. Anything inside that is fragile will require a protective container.

Even cheaper would be something like surplus ammo boxes, which are pretty darn tough in my experience, but they are sold as is, so you might be taking a chance.

Good luck in your search...