Well, since the outside shell is a ground, you could do the following:
1) use an old atomizer. Take the guts out leaving the metal tube and the connector on the bottom.
2) drill two holes in the side that are the diameter of the insulated wire leads
3) Insert 1 wire and solder it to the connector's center at the bottom (
through the hole filling the hole with the wire & solder). Sand/file it flat. making sure there is no short between the center pole and shell.
4) repeat #3 with battery connector inserted into top side and other wire. You will wand to solder the center 1st, then insert into tube.
So the wires connect the centers of the two connectors. The shells are connected via the tube. Of course, put alligator clips on the ends of the 2 wires. Make sure the holes are big enough that it doesn't cut
through the insulation.
If you don't use a metal tube (old atty) then you have to solder another wire connecting the outside shells together.
If you put the two clips together, it's an extender. You could measure amps through it via a meter in between the clips, for example.
This gives you an IN-LINE connector to the positive voltage before the atty/carto since there is a voltage drop across any resistor. Wow... sounds more complicated than it is. Just imagine the two wires with clips coming out the side connecting the centers.