Drives me nuts that (far as I can tell) nobody makes a personal-junk-sized dry box with any provision to tie it down short of running a strap over the top . . . which means you have to unstrap it to open it, which, especially if you're rowing, is a PITA. Here's my potential solution, so far not tested on the water.
with my pelican case I drilled and ran a long bolt through the two back fins. I then run a strap through the handle down under my frame and around the bolt. gives me great access and its going nowhere. I saw it done on here a long time ago so cant take credit.. on my little boat I have two ammo cans that I welded tabs on that are slotted so I can drop it in my frame on either side and have it strapped in with access. on my same boat up front I attached footman loops to the bottom third of 2 fat 50's and strap them to a side board.. all work great and are really cheap options. that looks pretty nice for ease of attaching and detaching..
Built rocket boxes to hang from frame very light weight.
I just picked up two of those plastic ammo boxes from murdochs.. the handles look great for strapping it down. seems like they would be splash proof but not waterproof.. I will test them out here before too long. what is your experience with them.