Recently purchased one of those bicycle lights using the XML T6 LED x3.

Came with a 18650 8.4v 6400mAh Battery Pack and a Pouch with Velcro.

The headlight has a voltage status function via a multi color LED but to be honest i am not satisfy with what function so i decided to add a voltage meter.
Furthermore, i decided to forget the pouch as a battery case and make a new one instead.

So, the new battery case it has to be cheap, durable, waterproof, nice, with some extra space and easy to work with.

The answer came from the plumbing tubes!

I have included a lot of pictures, so the whole assembly process is fairly easy to follow.

Step 1: TOOLS NEEDED

For those who wish to follow me on this project here is the instructable and the materials needed.

Step 2: COMPONENTS NEEDED

x1 female plumbing tube 10.5cm in length x 50mm in diameter with O-rings seal at both ends.
From Praktiker at about 1.80 euro.

x2 male end plumbing tubes 50mm in diameter.
From Praktiker at about 1.80 euro in total.

x1 LED DC3.0-30V Blue Voltage Meter Display Digital Voltmeter Self-Powered.
From ebay at 2 euro. See here.

x1 Micro JST 1.25 2-Pin Male&Female Connector plug with Wires Cables.
From ebay at 0.24 euro. (4.72 euro 20 set) See here

x1 Silicone Tailcap for Flashlights 14mm.
From Deal Extreme at 0.15 euro. See here x1 Clicky Switch for Flashlights 12mm.
From Deal Extreme at 0.30 euro. See here

x1 clear acrylic sheet at 5mm or 3mm width with a 14mm hole at 8 euro from local store.

x1 Velcro strap at 1.5 euro

UPDATE:
1x shrinkwrap tube = 10cm in length x 16 - 20cm diameter for the battery.
2x shrinkwrap tube = 3cm in length x 14mm diameter for the JST micro connector.
2x shrinkwrap tube = 15cm in length x 6mm diameter for the JST micro connector's cable.
All the shrinkwrap tubes will cost about a euro.

Total cost: about 17 euro.

Step 3: Sanding and Painting Preperation

Sand all tubes using a Nr.400 sand paper.
Then use a modeling knife to cut as shown in picture the holes for the volt meter and for the Silicone Tailcap.

Step 4: Painting the Tubes

Apply at list 3 layers for the color paint following the spray can manufacturer's recommendations.
Apply at list 3 layers for the clear varnish following the spray can manufacturer's recommendations.

Step 5: Glue the Faceplate

Step 6: Disassemble the Battery

Step 7: Prepare & Connect the JST MICRO Connector

Step 8: Connect the Manufacturer's Wire and Shape the Shrinking Tube

Updated pictures:

Step 9: Prepare the Circuit and Assemble the Parts

Step 10: Assemble the Tubes

Connect the female JST connector from the battery to the male JST connector from the circuit.
Assemble the tubes by pressing the male tubes to the female tube.

Step 11: It's Time the Place the PowerTube to the Bicycle

Use a Velcro strap to hold the PowerTube to the bicycle and connect the light.
You are done!

Hope you like it!

UPDATED at 19/5/2013 ( pictures and materials )