Here is how I made my new garden shed window box using an old copper boiler.
I drilled several small holes into the bottom of the boiler for drainage.
I didn't want the soft copper to tear so I used a piece of scrap wood to attach the boiler to the shed wall, and help distribute the weight.
Since the copper boiler is deep, I added several 2 liter plastic pop bottles to the bottom for filler.
I added miracle grow potting soil on top of the plastic bottles and planted my flowers. Now I'm just waiting for them to fill out and get lush.