Be sure to put your rain barrel above the height of the garden even if you have to build a platform for it to stand on. It can be hard to get the last bit of water out if it has to go up hill.
Does it sound like I've had some experience with failure? Hmmm...
Haha great advice, thank you! I am thinking of running some form of guttering around the fence to catch more of the rain as it sits on a downward slope from the back of the garden.
The gravity feed will definitely be high enough and (Bonus!) you should get a nice big surface area to collect in without having to dedicate a large plot of otherwise usable area. Nice idea! Lots of luck!