Not quite sure I understand your intention. Are you thinking of using the lid and flange as a hatch cover to a knee tube accessible from the deck? If so, you might want to leave 1/2"--1" or so of the body of the container sticking down below the flange. If you then made a flat piece of wood (stripped together or whatever) and cut the hole very snug fitting and bedded the flange in something like Sikaflex or Marine Goop, and maybe passed a few SS or brass bolts through the flange all around, that'd probably work pretty well, long as the sealant will stick well to the plastic.

If you're talking about cutting the flange off and using the body of the container, then I don't understand what you're going to do with that. How will you seal it?

And if you're talking about using the lid in some way like I described, other than it being readily available, you might be better off getting a small access plate with an o-ring from a marine store, one that screws on. Don't know where you paddle, but I could potentially see surf taking the lid of a tupperware container off.

Throw in a couple of those dessicant silica bags that soak up humidity to help with the inevitable moisture from reaching in with wet hands or just condensation.

Would a medium size Pelican type box with a big patch of velcro on the bottom to sit on the floor in front of your seat work better as an in-the cockpit place for electronics? I wonder, because I've used a small tupperware type container to hold a mini-tackle box full of hooks, sinkers, swivels etc. One day when I wasn't paying attention to the tide a wave washed up to my fishing gear on the beach. I found my "hook box" upside down in the sand. The lid was on, but it was also about half full of water. Maybe the lead sinkers dislodged the lid a bit when the box tumbled, or maybe the seal was dirty, I don't know. Maybe it just didn't seal well.

: What I have in mind is to take this plastic container and cut it off just
: under the flange and silicone that to strips that run about a foot in
: length. That way I can have a water proof compartment. The one I picked is
: small but I can get get my camera and GPS in it plus maybe a monoctular.

: John