I'm living with the internal bungies ,with careful placement ,not so bad.I don't like the look and splashing(in my face)of the external "clamping"methods.Magnets is beyond my simple get it done methods.However as for shape,I like to compliment the curves of the combing ,hull and deck,etc so would be reluctant to be looking at square hatches.The corners would have to be radiused enough to take the bend in the upstandiI have on the hatch lip to penetrate the gasket seal .Because it is an anxious moment ,cutting into the deck,I consider these overly well and make sure that my equipment can pass through and be stowed.I also consider how the the hatch would locate in the hole.I don't want to be looking at the dam thing twisted with all the decking not being continuous,so the hatch shape is always elongated ,not circular,with edges parallel to the shear-line,or just straight, and ends that are complimentary ,aesthetically,to the big hole in the middle.I do ply laminates with a spacer deck and hatch so
I can be clumsy with them and I also glass ,bias cut,around the raw edges of everything with real light cloth.
They leak in as much as the amount of water that sits in the system apon opening them but I always have everything in dry bags for the inevitable day when they blow out or the hull gets crunched,badly...