KWRANCH:
Fantastic concept - How about lining that thing with some neoprene or something to keep that gatorade cool? I have had horn bags that lend stability by having a wider attachment piece to the horn. Always thought a dee ring & leather lace at the bottom that could clip into either a breast collar clip or rigging would also keep that beverage from getting flopped around.
As far as selling price, please factor in your time. Is it a piece of art or a paycheck or something in between? I'd give $15 bucks for one and wouldn't care if it had tooling!
Hope this helps!