Just to add to my previous comment, I think a very simple method that would work great for a person who can’t or doesn’t want to build anything elaborate, would be to simply bolt short lengths of webbing to the box, though which your tie-down straps on the boat would be threaded. Burn the ends of each cut piece of webbing to keep it from unraveling, and burn a hole near each end through which a bolt will pass. I’d put these at the bottom outer corners of the case, with the exposed hardware facing out so you can still set the box down without scratching whatever it’s set on. In this case, since the webbing is somewhat “squshy”, I’d set up the anchor hardware like this: I’d put the bolt head on the inside, with a small washer. On the outside, I’d place another small washer, followed by a nut, and that nut would be very tight. Within this part of the connection is where your sealant would go. Then, on top of that first outer nut I’d place a bigger washer, then the end of the webbing (via the hole that you burned into the material), then another big washer, and finally another nut, which would be quite snug but not extremely tight. That last nut should have a nylon locking insert, or you could stack two low-profile nuts, with the second one torqued tightly against the first (that will lock them together without increasing the clamping force applied to the webbing). Then trim and file the end of the bolt flush with that last nut to remove extra material and sharp edges. If you used very small bolts (roughly 1/8th inch in diameter would be good) and “thin” nuts, even this stacking of three washers and two to three nuts along with the webbing thickness would not protrude very much from the case.
You could skip the first washer and nut on the outside of the case, but in that case I’d use some kind of hard gasket material to supplement your sealant beneath the washers, both inside and outside the case, since that part of the connection would likely be less tight. This method would provide a lower profile for your attaching hardware.