well, my original idea for my bog was the exact same thing your talking about, the 4 foot under the bed storage bin. But, i planned on having 2, one sitting on top of the other(bottom bin with lid on) and having the bottom one be the reservoir. it still used these same fittings i showed you and allowed it to fill and drain at one level and if something were to happen to the pump, the upper bin would still hold about 2-3 inches of water.
i might be crazy on the whole reservoir thing but i really dislike having to keep filling 1 gallon jugs multiple times everyday.
....maybe the bottom bin could even be buried to help keep the water temp?