i built one and nailed it all together then relised as i improved that i had wasted my time!
because i had built it in a kind of straight line so when i improved that was it, it was to easy for me so i knocked it down.
my advice is that get a shit load of palets in different sizes of piles ect logs , railway sleepers, concrete blocks/ pipes
any loose items that have a low centre of gravity
hope i helped