Wheel arche rust fixing help!

Hi,

I'm currently fixing the rust on my G wagen and whilst fixing I came to a part where it all looked good etc and i hit it with a hammer to check it and it was fine. i then when to clamp a bit of metal to it so i could patch it up and the whole thing sort of caved it leaving a strip by about 1" X 2ft.

The part is on both rear wheel arches. I can't take any pics as my cameras broken :( so i'll do my best :)

Its up in the wheel arch heading towards the front doors and where the rust has collapsed it is all hollow so I can't push against anything to fix it. I don't have access to a welder so are there any other ideas to fix it or should i continue fixing the rest then pay someone to weld those parts?

many thanks
Joe