Curved ramps arent all that hard to make, it just takes a little bit of work and time. I used to make them by cutting the side pieces with a curve, and then running 2X2 cross members, and then screw the deck down a little at a time until it conforms to the curve. Some people i know would wet the plywood, or run notches every 1/2" inch to help it curve, but, i never did any of that. All i ever did was screw the wood to the bottom cross brace, then, stack cinderblocks on the deck for a couple days until it fit the shape i wanted, and screw the deck to the cross braces once it bent enough. I only ever used 3/8" or 1/4" wood for the deck though, and 1/2" for the sides.