I use this one and the metal cap also fits on 100ml bottles I mix in which makes it super handy. FT also sell a 5 pack of these too. Like @Rob I couldn't imagine going back to the faff of refilling without one
Up until i got my topside i was using one of these to carry/fill liquid. Without this i dont if i would have lasted that long squonking
Just to add, be careful not to accidently hit the fire button when filling with this bottle. As you'll short out n cause all sorts of issues. I believe some bottles either have a plastic tip or some sort of insulation to stop shorting if u hit fire, however the one above does not have such protection.