I just made 2 coils from some old transformers. It took some trial and error, but the end result was good.
The spark is just as good as on my CH ignition coils, and the weight is the same, about 40 grams.
The size is also about the same as my CH coil.
I have tested a lot of different ratios, and several ways to wind it, and this is my best results;
Primary: 100 turns of 0,8 mm wire
Secondary: A LOT of turns of very thin wire. I did not count, but the layer of wire was about 5 mm thick.

A short clip of it running is here: [link=http://home.online.no/~jon-mj/Coil.wmv]Coil clip[/link]

The images show the final coil, and a side by side comparison with a CH coil.

I will write a description of how to make the coil later.

Jon-Magne