Sounds like an easy enough project. I'd guess a couple 2x3's, some plywood, and maybe a little carpet for grip and you're in business. Perhaps some rubber on the end that hits the ground for grip, or make sure you offer a planted foot while the dog is on the ramp to prevent it from kicking out. You could even add some heavy duty hinges and make it foldable.
Here's lot of pics to give some ideas...http://images.search.yahoo.com/searc...as&fr2=piv-web