I sewed a new bag out of old jeans, inspired by a picture on Pinterest. The bag is lined, with recessed zipper, adjustable strap and one inner pocket.
For the back I preserved the seam of the jeans, an old pair of Alex's, which he outgrew quite fast, so it was barely worn out.
The recessed zipper gave me quite a headache, until I found an youtube tutorial and figured it out.
The lining of the bag has reinforced denim bottom:
Some process pictures - the zipper (I replaced later the tab on the zipper with a smaller and more unobtrusive one)
One piece of the lining with the bottom and the pocket
The inner part of the bag:
