I have seen those in the local surplus store. The price is right but I could not find one in that store that actually seems to seal up tight. All of the rubber seals were shot I think. Or missing completely. It was metal on metal and there was clearly some play in the system leading me to believe it wasn't going to be dry at all. maybe splash proof. The latches did not snap tight, which is unacceptable.
I thought about getting one and finding some rubber stripping to seal it with but in the end that felt a lot like work and the result was not assured.
Then I thought more about the design of the medical cabinet and thought it would be hard to get it full with such a deep lid. You would have to carefully stack stuff and sneak the lid on I think.
Ultimately I went with a couple of frontier play dryboxes. I have been happy with them for the price. bone dry so far, though the softer outer weather stripping needs replaced after 2 seasons and 30 days of use.
Unfortunately it looks like 26 would be the only size that would fit.
I have about 2' of play between my cat tubes/frame and boxes. they do not seem sloppy to me. They never shift. I understand maximizing capacity but for the price I paid combined with my tendency to pack light I have been happy.
If this attachment works this is my entire gear load for a 5 day trip in the fall. 15' cat, frame broken down, 2 dryboxes, 1 pelican, 1 bucket with fishing gear that I should have left home.