I have two videos showing the construction of a compound that we can keep our trailer in and keep it dry, a few months later we now have plants and a cat in there he hardly comes out !
The gate was made later, just by screwing a few of left over board together with two horizontal brace and a diagonal brace to restrict wood movement.(shown in image 9).
The videos may not show the full progress as I only started filming part way through but I hope it serves as a good rough guide. Enjoy !