.... exactly, that standing water will ruin everthing! If u put an inch or two of perlite or pebbles in the bottom it will help.12" might be sufficient but 18" would be even better. Make sure it drains well so you don't have standing water in the bottom.
This is the best idea; just go with some big square containers and it will be much easier.just go with some standard containers and take away the guess work