Other material to consider making it from is an old plastic oil container or an old 25ltr oil drum (if you want a really big one). Cut to shape and rivet, or nut and bolt using big enough penny washers on both sides.
Make it out of cardboard first, to get the shape right.
I think you'll need to put a decent flange on it. Use a heat gun or blow torch to help you get the fold then chill it in a bucket of cold water.
Here's one for the front I made earlier of my KE.
