I takes some design and thought. plywood curves pretty easily compared to lumber.
I though of an idea of curving the plywood, drill 2x4s about 4-8 inches apart running parallel to the end of the lip from the end to the base.
secure those together with a bunch of shorter 2x4s parallel to the rail connecting 2 boards at a time (have 3-4 across for every board).
elevate the curve to your wanted dimensions, put 3 to 4 height supports across (depending on width of lip).
for extra security have 2 2x4s on the outside edges of the lip, secure one end to the top of the lip and one end to the base.
If I could sketch this out I would but I'm terrible at sketching.