I would look at craigslist or your local paper for a used trailer. There are lots of trailers out there that sell for way less then you could ever build one. Then, if you really need bigger tires or the like you can modify it. Harbor greights also sells a very small trailer and it is an easy to build kit but understand cheap kits give you what you pay for!
Also, you do want some type of suspension. If you don't have basic springs when you are driving down the road your trailer may bouse 2-feet in the air on a bump and your ATV is going to take the abuse or your trailer will just fall apart unless it is built like a tank.
Here is the harbor freight trailer, 870 pound version:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=42708
Bigger 1090lb load version:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=90153
For $200 - $240 you would be hard pressed to get everything you need anywhere else to build a small trailer and this says it can haul 870-1090 pounds! I have their larger folding trailer but we don't use it very much at all. Just note you may want to weld the joints or at a minimum double nut all the bolts as the bolts work free while driving these bolt together trailers.
Also, check how much it will cost to get the dambed thing inspected, titled, and registered. My $300 trailer cost me about $400 to get titled so I could have just bought a pre-built trailer and made out better if I valued my time at all. PA has some really painful rules to put a trailer on the road so your state may be better!
This is the one I bought... and it does work fine for a cheap trailer I just was poorly prepared for the cost to get it inspected, registered, titled, etc here in PA....
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=90154