That's awful. I had an 02 Primera and when I part exed it two years ago there was no rust on it whatsoever. I only washed it about twice a year.
A handy way of making a template is to cover the hole with strips of masking tape, then rub round the metal edges with a pencil to get an exact replication of the hole. You can then stick it down on your metal sheet and cut round it. Doesn't work so well for curved bits though.