Hey Dreamtime
We are in the same situation as you. My wife and I got our 42 this last June and are also needing to convert to propane. Our original thought was also to put it in the starboard deck locker but I ran into the same as you, how to get a double box into the locker. Since I am over 1500 miles from the boat right now, I am not able to measure and see if a propane locker for a single 10 lb tank will go in. If it will we are thinking about installing a locker for a single tank and then rail mounting something to hold another 1 or 2 tanks. Please let me know what you come up with so I do not have to recreate the wheel!!
Thanks,
Tom