I love to work with iron and metal. But, there are many troubles to work with them. First they are heavy and how to glue them on a surface? The best way to work with this kind of stuff is to make "fake" rusty iron.
What you need:
raw umber acrylic paint
iron rusty kit
pencil, cissor, etc.
To make the envelope use one as template. Lay it over a cardboard sheet, cut it out and glue it in the form of an envelope.
Paint the whole surface with the acrylic paint. Let it dry and apply the rusty stuff. Let it dry again. With my brazilian rusty kit the more I wait, the better it will be. Easy, doesn't?
So the fever of rusty things take me and I have painted some coasters, and little pieces of resin.
The leg I made with Fimo and used the same procedure above.