Yes, that locker looks the business, but if the box is in the cockpit locker, to get the bottle in and out of the box, won't the box be below the waterline? And if so, won't the box fill up with gas before spilling over the side?
I'm in New Zealand, by the way. Took over Skye in a very run down state about 5 years ago, and lay her up each winter to continue the rebuild with lots of personalization. I'm over 3/4 finished the work, but the usual story with boats - longer and more money required than anticipated !!