This is a simple and short instructable to make a cable bike lock/bike lock extender from a disused laptop secutiry lock cable, like this.
A number of these laptop security cables were donated to my local hackspace in Nottingham UK (www.nottinghack.org.uk).
I needed a cable extender for my bike lock, which can be used to lock all the wheels (similar to this kind of lock).

The problem with the laptop security lock is that only one end had a loop. To make it useful I needed a loop on both ends which could not be undone.
So this is the technique I used to create one....

Step 1: Parts Required

You will need:
Laptop secutiry cable
Section of thick-ish walled steel tube (I used a section from a childs bike frame)
Self-amalgamating tape or large section heatshrink

Tools required:
Scissors
Hacksaw with metal cutting blade
Large cable crimper (needs to be around 120mm2). This can be either handheld or hydraulic. You will only need this for a minute or so so maybe ask someone who does battery cable lugs?..

Step 2: Cut Tube

Using the metal hacksaw, cut a piece of steel tube. I used a scrapped steel bike frame which was about 2mm wall thiuckness and around 25mm diameter. It must be wide enough to have two runs of cable going through it, but also small enough to fit into the crimpers.
I cut around 30mm long piece. I was trying to match the other end, but this was a test so I didnt measure it too well....

Step 3: Crimp Loop

I cut a peice of heatshrink long enough to cover the tube and placed it onto the cable. (In the end I did not use it - I used self-amalgamating tape, but this is in case you do).
Then place the tube onto the cable.
Loop around the cable end and feed it back through the tube.
Pull it through to ensure the loops are a similar size.
Place the tube into the crimpers and pout one crimp towards one end of the tube.
Depending upon how long the tube is, put more indent crimps one it.
I ended up using three.
This should be pretty darn secure - someone could cut through it, but it would be a big hassle.

Step 4: Cover With Tape and Finish!

Either: slide the heatshrink over the crimped tube and then shrink with a hot air gun (not shown here as my heat shrink was not quite large enough and I had to cut it off).
Or: Wrap with self amalgamating tape (the type used by electricians to make waterproof joins).

Now your have finished and have a pretty sturdy bike lock.
I would not use it as my main lock, but to quickly attach a front wheel to a decent D-lock then this would work.
Pretty good for free...