The complete emergency kit might take up your whole trunk if you have a smaller car, so if you want to make a basic kit, pack up two roadside flares, a quart of oil, a small first aid kit, extra fuses, a flashlight, a multi-purpose tool, tire inflator, some rags, a pocket knife, pen and paper and a help sign.
You could also go with a pre-assembled emergency kit. Some options are RightTrak's 58-piece Deluxe Auto Safety Kit ($24.00) and the 78-piece Auto First Aid Kit from Home First Aid ($39.95). While these kits contain the basics in a small convenient carrier, you might want to add a few more items to get exactly what you'll need. Compare the contents of the kit you buy with the lists provided here. Get as many things you feel are important into the pre-made kit and you should be good to go for just about any road mishap.
Unfortunately, there isn't "one tool for all roadside emergency needs." But with a little planning and a smidgen of trunk space, an emergency roadside kit can often save the day.