Not modelling filler, Daniel, you'd pay so much you'd be able to buy a share in a real Corsair!! The corrugations don't look too bad, so you might be able to tape over them, maybe with Duct tape or similar, then fill the remaining recess with a car-body filler, the 'elastic' type that won't crack. Your nearest auto-parts/ accessory shop should have the right one, and advise you which is best. If the corrugation ribs are raised, tape up to them first, to give a smoother look, then do the filling, to blend the whole lot. Remember to take weight into account though. If it still doesn't look smooth overall, the only solution I can think of at the moment, is to then cover the lot in fabric, like they did with the Mosquito, over the plywood. You don't have to buy expensive linen canvas, just use something like cotton, or even polyester bed sheets. Glue it at one edge/end first, let the glue set, then pull it tight around the skin, maybe spot gluing as you go, to join-up where you started. If you make the start point/joint UNDER the fuselage, it won't be seen. Then, give it a coat of thick varnish, as if it were dope, and let that harden. Paint over that and presto! Should work. I hope!