Doing a diff is going to be messy... you fill and fill until it dribbles out.
If youre picking the vehicle up a bit, or have lots of clearance, then either using a 'squeeze top' which is a point with a hole in the end, as is found on most diff fluid bottles is the easy way to go, and nothing is needed. if your lube bottles dont come with said pump, and you have clearance, than a 'soap pump' type unit is the best.
For on the ground installation, with little clearance, I have found that nothing beats the tubular 'suction gun' where you pull the plunger and then force the fluid out the plastic tube. Yes, its a bit more difficult, you need to be more deliberate in your motions and watch the orientation of the tube end... but IMO its the easiest way to go. Dont fill it all the way.
JMH