Arc is not normally recommend on cars but in this case you might be lucky due to the thinkness of that metal.
1) First cut out the infected section, normally in a square shape with you angle grinder. Cut out a little bit more than the rusted section.
2) IF you have the same type of metal with a simliar thickness great, if you don't go down to your scrap metal dealer he should be able to get you something to suit.. shouldnt cost more than $10-20
3) Take that square you cut out and trace it onto the new metal and cut out the new metal, cut as close as possible.
4) Weld a peice of metal,bar tube whatever to the face of the new metal so you got something to hold onto when trying to place the new metal in the hole.
5) Weld the new peice in , make sure you using the correct arc rods and amperage( can get this info from the google :happy: )
6) Grind to clean up the welds, treat metal with anti rust stuff then paint.
edit : OR just buy a new 1 from a wreck