1/2 Gal jugs are easy to find. A good base plate and strap situation is where the value lies here. A bit of hose and bit fitting are no big deal. I used a dog dish for the tray, bolted it to the foot peg and bought some food grade tubing, done. It's ugly as hell, but works fine.
G, make up your base plates to accept a rok strap, trim those to the ideal length, do the tubing and bite valve and you're in like flynn. Bonus if you want to sew up neoprene covers to better insulate the jugs, but as you said, it's not that big a deal.
The value lies in a quickly detachable system so you can fill it with ice and water at stops. And the security of the system so it doesn't fall off.