Cut back the rusted sections, much of it will break off with a little pressure.
Sand to bare metal and spray with phosphoric acid. let it dry, then wash, rinse, prime and paint.
Use scrap metal to mock up replacement and pop rivets to secure it. Paint patch to suit.
Easier than it sounds actually.
If you have or know someone with a welder, you can weld it on, in which case you can skip the prime and paint of the metal under the patch.
There are a lot of sites that detail how to make patches on car bodies, with different goals. some are strictly for looks, other stress function, others are for good looks and solid repairs.
The most difficult part with that repair will be access.
If you just want to stop the rust, cut off the horizontal parts, wash it out real good and spray with phosphoric acid, it's normally sold in auto stores as 'rust converter' , but can often get it cheaper from pool and garden supply shops. Then when the acid has dried, paint the interior.
It will keep mud and dirt from caking up in there and slow the rust.