In reply to neuromancer:
If you find one let me know, they are handy for sticking in pocket to fill up while skiing in resort without getting fleeced 8 euros for a 500ml bottle.
I actually made one for a platypus once that came with a bladder, i cut a disc of plastic and put it inside the cap, seemed to work OK, might see if i can mod one of my older Salomon ones to do the same.