I'll suggest first that the lid frame have a slight pitch to it for drainage. I'd think that 1"-2" taller in the center would be enough. Make it a gentle radius at the pitch line if using a composite for the skin. Most large steel vendors can roll those straight tubes into a arc of whatever large radius you might want.
Steel can be welded on, anything else will have to riveted, bonded, or riveted & bonded on (what I'd do). For bonding I'd look into the Silkaflex products and 3M's VHB tapes.
0.03" thick aluminum is available in rolls for building circle track race car interior 'tin.' This comes clad in various colors, so painting may not even be necessary.
"Seaboard" was one FRP panel trade name that I ran across years ago. This was a panel like the current smooth sided RV's use available in various sized sheets. No idea what current availability or trade names are out there.