My friend built one out of plywood, then coated with fiberglass, and carpeted to match the boat. It looks like it came from the factory. He said the fiberglass was the hardest part, but the overall project was fairly easy. It was time-consuming though. You have to allow for the thickness of the applied fiberglass and the carpet.
If all you want is a step, consider having a piece of aluminum tread plate mounted in one of the corners. It makes a convenient step and doesn't take up any floorspace. Look at the G3 boat as an example.
http://www.g3boats.com/GatorToughDel...072CCDLX#model
Here is what SeaArk uses in their boats.
http://www.seaarkboats.com/accessory...id=1&accsid=45
Good luck.
Joe