Step 1: Material Used and Preparation
2 PE caps from water bottles;
8 tie wraps;
some wire from an old PC flat cable;
1 kit for a female JST connector;
using scissors remove everything but the inner portion of each cap and make 4 buttonholes at 90° each other on the top just close to the rim. These buttonholes have to fit the width of the tie wraps, you can obtain them with a small screwdriver or the tip of the scissors.
Punch another hole on the side of the rim to let the wire go inside and solder the washer on its end.
Repeat the same for the other side.
Step 2: Install the Battery and Tighten the Tie Wraps
Cut the head out of other 4 tie wraps and use them to tighten the tie wraps already installed on the battery holder. To ensure a good electrical contact use the natural springiness of the cap that is going to bend a little bit around the washer.
Trim ties in excess and (optionally) also shorten the heads on the other side of the holder.
Install the JST connector on the wires according to the polarity of the battery, fix the wires with some duct tape to avoid their ripping.
It's done! :-)