5-gallon(ish) water jug with separate fill hole?

Since you said you could make a custom bracket... Coleman has a square 5 gallon jug as seen in the above link to Walmart. You could build a rack where you flipped the jug up (empty) to fill it, then lower it (full) to use it. At $11.39 fits your budget and looks like the jug was designed to fill from the top then lay on its side.