There are a lot of people here designing power supplies. Probably to sell . Unless your gifted at power supply design you would be better of buying one. Because by the time your done making one you will probably spend more then it would cost to buy one.
Although if you want to try dont listen to me. I spent about 40 bucks building a 15 amp regulator from scratch. I should probably modify it to 20 amps like i started out to build.
Make sure if you do buy one you get a good one. The pw200 is not regulated at 12volts, i had to find out the hard way.
If you do find a good schematic on building one let me know. I will spend another 100 bucks for fun. I'm so close to signing up for a power supply/electronics class at the local state college so i could design a kick *** power supply.
Anyways, good luck