There's ALWAYS a Plan B.

Heath Robinson Mk ll.
Way back, here, I started to sort out a water system for the greenhouse.
It has not taken this long to perfect; it just struck me I hadn't updated it since.
Plan B was a sight easier than Plan A; which was to lift & lower the inner drum as level rose/fell in the outside Butt.
I made the hole I promised. (Couldn't do it with glass)  ;¬)
Thus negating the need for going through the door opening.

Plan B was the tap - a lot easier than upping & downing the inner 5 gals, which, at 8 lb to the gallon, meant yer actual work, this doesn't. 
I'm pleased I checked, I remember, from Fire service Hydraulics, water as being 10 lb to the gallon, so 8 didn't ring true. Without specifying, of course I got US gallons. 
WHY does such a HUGE land mass have such tiny gallons?
This, also, allows for a pretty accurate depth gauge too by lifting the tap to the point where flow ceased..