There are quite a few methods posted on building a 12v bicycle light.
And I to have build my version,
however I thought I share the simple way of transporting the battery on the bicycle.
My needs were:the method had to be secure,I do cross country at night
and the battery had to be easily accessible for recharging.

Step 1:

Water bottle battery holder
First step get a battery and water bottle the 12v battery I use is supplied for home alarm systems.
The water bottle an inexpensive one.
You will also need some foam to seat the battery into the bottle and a little piece off webbing.
I have also used an off cut of neoprene to keep the bottle locked when assembled.

Step 2:

Measure the battery against the bottle and then leave about 3 cm of space above it.
This is the line,you now cut the bottle.This forms the bottom half and the battery wrapped in the foam is slid snugly into this.The webbing must be used as the method of getting the battery out again.
Just run the length of the webbing around the battery before inserting so it leaves the ends like little handles.

Step 3:

The cable with the connectors are fed through the mouthpiece of the top half of the bottle.
To get a nice fit I attached the terminal connectors only once I had pushed through the cable.

Step 4:

Now connect the power cable to the battery, slide the battery into the bottom half.
With its foam packing. Coil/role the excess cable into the space above the battery and push on the top.
You might have to cut a little slit into the side so the top half can compress and slide into the bottom half. The neoprene band then covers the join as well as holding the ac conecter,insulating it against accidently shorting on the metal bicycle bits.