I use this for my camp bottles, there is very little chance of overfilling
(IMHO) since the bottle gets only half full of liquid. It takes some time to
drain liquid into tank.. not just a few minutes  or you'll get less than 1/4
bottle

and as for transporting tanks inside... then they shouldn't let us cart them
in our cars from the hardware store.... those are full and "heaven forbid"
maybe over so   .. just please keep in mind of them in a HOT car on a HOT
day

Refill Propane Cylinders in One Minute or Less!
Solid brass propane filler coupler lets you fill refillable propane
cylinders FAST from your 20, 30 or 40-lb. propane tank. No need to buy a new
replacement cylinder when you can refill it in a minute... with about 30
cents' worth of gas from your large tank. For refillable cylinders only.
Ship Wt. 1.00 lbs.
Item# 17264
http://www.northerntool.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=11829&
prmenbr=6970
 feel free to ask or advise. I listen as well as learn.