this is how i make all my barrels now.you could do the same thing in any of the buckets in your system and then hook a hose up to the shut off valve and connect that to a utility pump to pump out the system.i have three barrels like this with 55 gallons and no leaks.View Image
hell ya.
i end up using plastic milk crates an screw down a piece of 1/2in-3/4in plywood to it.
for well over a year i got 2 milk crates holding up a blue collapsible autopot 100 gal top off rez with no problems. 800lbs of water. thats 400 lbs a milk crate. no problem
i drove thru some ghetto action a few months back. seen a car with wheels removed on 4 milk crates..... now that was impressive.