In the vast majority of rusty lower cowls the front area is normally quite OK . I leave that part intact and cut up vertically just behind this area . At the very back vertical return this is normally OK as well . When the front portion of the patch panel is removed it is far more flexible and it is easy to mold it to the compound curve of the cowl . If you make some spaced cuts in the bottom edge you can then bend the patch panel slightly to make the correct fore and aft curve . As your patch panel looks like the dead flat type and not die stamped this is the ideal fix , Just MIG the cuts shut when done . The pic I posted is of a homemade patch panel on my AA truck ,never got around to do dolly work and filler as other projects put it on the back burner . I have used a "flat" panel on my USA pickup in Sunny Chandler Az and used this system and it turned out fine . Also you know your cowl band will fit a treat !!!. The top picture is the homemade panel after the welding was completed ,it still had to be dollied and filled but you can see better where the welds are ,I put primer on it to protect it so I could get around to it another day . The second pic has the same welds on my AZ pickup ready for topcoat .
John in overcast cool Suffolk County England.