Some years ago I stood next to a girl with an interesting shoulderbag on the bus. Upon closer inspection it turned out to be made from a pillowcase and some patterned fabric for the strap. I really liked the idea and luckily had just the things at home to make my own.
The body of the bag is a velvet case for throwpillows, the strap is an old bathrobe belt and the flower had been waiting in my embellishments stash of a new use.
Since the pillowcase and the belt were already sewn, all I had to do, was attach the strap to the body of the bag. I used safety pins and some trial and error to determine the most practical length, then ripped the seam of the belt apart as much as I needed to fold the material around the sides of the case. It's best to used small stitches when sewing on the strap for stability. The bag I saw on the bus had a wider strap, attached down the entire sides, which is probably even sturdier, but mine also holds up quite well.
When I made this bag, I thought the silkflower would add to the hippie touch, but it really suffered from washing over the years and was difficult to sew on, so I can't recommend it. Maybe embroydery, applications or some removable decorations might be the way to go.