If you go the plywood route, there is marine grade plywood available. It uses different glue to stand up outside better.
Don't forget to paint or stain or seal both sides, otherwise the painted side will peel after a while.
The only reason I wouldn't use plywood is that if you get a gouge or scrape, it will show the next ply, with grain going the other way. If you use solid wood, you can sand, reseal and it's repaired.
If going the solid route, I would groove the sides with a router and install splines to keep the boards level to each other and prevent road moisture from getting in from underneath. Similar to tongue and groove, the spline acting as the tongue. Renting a surface planer to ensure the boards are flat and uniform thickness wouldn't hurt, as well as a jointer to ensure the sides are a good fit.