Insert both sides of the cable into the box and have them ready for connecting.

Use some cable ties to secure the cable inside the box. You would have to use the 2mm ones or even thinner.

Use some heat shrink tube and place it on tot he wires before soldering. After joining the wires, heat the tubes to shrink them:

Use a hot melting glue gun to fill the box and the space between the wires. Inject the hot melt until it squeezes out through the holes. Do not inject more than the lid level. 

Place the lid of the box when the hot melted glue is still liquid.