There's no built in method for securing it - under normal situations its fine. We were going through a channel and had rough surf causing the stern to get low enough to the water to catch and flip the dinghy enough to lose the gas tank. It's not as if we found it upside down, it righted itself, but tipped enough that the gas tank fell out.

This was not our first time on this boat either - just the right circumstances for this to happen. The only mistake made is that we probably should have removed the gas tank all the other times as well.