Poke all of the areas from the inside with a screwdriver until the hole is as big as you can get
strip all of the paint off the outside area plus an extra 20cm in every direction
cut out the area that is littered with holes and any other thin area you can see from the inside.
weld a piece of sheet metal in. If you don't have the right shape, an old damaged door off another vehicle may have enough area that is undamaged for you to cut out and use. It will also give you metal of the same thickness and a slight curve.
tap it up nice and smooth, fill, prime, putty, paint
treat the inside with rust converter, prime, paint and coat with fishoil, tar or whatever you decide on.
