As mentioned earlier in the thread, plastic battery boxes do make great dry boxes......I made one up for the young BIL boat a few years ago.
you will need a couple of plastic hinges and one of those elastic tie down thingies that are used on some torneaus and other stuff.
to get it to work you have to rivet the hinge to the outside of the box from the outside and to the inside of the lid from the inside (you could use cs screws and nuts).
rivet the elastic tie down to the lid and the knobby thing to the front..and you are almost done.
put some sticky tape over the vent holes at the to and silicon them up from inside...peel the tape off when finished.
now you can just screw it down wherever you like.
One final thing, throw a square of gutter guard in the bottom so if you do get any water in your stuff is not sitting in it.
Large BIL has had a pair of mec vapour gear housings screwed either side of his rear thwarts...these are the old cast ones....waterproof and lockable with a padlock.
Big brother makes up gunwal savers by splitting up 50mm polly pipe down the length......great for little boats.....takes some getting on.....leave it in the sun helps.
One of the single handiest things on a bloke's boat is a pissing pipe.
Simply a 2 or 3 foot lenght of suitable diameter plastic pipe......saves falling overboard or dribbling in the boat...
cheers