You can see that he has attached a gasket material to the carcass of the box, that will seal it real well. It looks almost like the adhesive backed weatherstripping I used for my box. The weatherstripping is a great sealing gasket.
My understanding is that if you have a vent to the exterior and it's located towards the top where the hydrogen could accumulate, the transfer to the outside will be taking place without a separate intake. I will probably install a separate, small intake in the lower part of my battery box but will cover it with a flap of soft rubber so that it can't ventilate to the cabin. Although I think it's unneeded for the hydrogen venting, it should exchange air better and allow for warmer cabin air to enter the box in Winter. A separate vent for entry air from the exterior would probably also increase the air transfer.
BTW, thanks for the link to the hanger bolts, I was just thinking about them, yesterday and had forgotten the exact name. Elegant idea about the sliding tray, too.