I'm holding it down by the edges to demonstrate that I don't have to hold the lid or anything, it really is watertight as advertised, and rated for 100 feet deep. As I'll only be submerging it 8 feet (perhaps 7 to be safe?) I have complete confidence in this thing.
Next up I'll need a power drill, so I can drill some holes for the aquarium air hoses. I'm considering two adjacent holes, one where I insert the tube till it touches the other end of the enclosure and another for a tube that barely pokes in; that way the air will have to traverse the entire interior as it's used, and the air delivery/return hoses will be close enough together where they exit the enclosure that I can wrap them around each other/tape them together into a single umbilical.