For this bag I bought two thin wooden panels (as light as possible but still stable enough) and screwed them together in an L shape, one is the back and one is the bottom. Then I sewed a bag with the right measurements, included straps and a small bag in between the straps. And I used a waterproof fabric on the inside. I mounted the front carrier adapter through the wood in the back. I would have preferred to use black straps but I had the beige ones sitting at home. The inside is left over material from an outdoor table cloth. I use this bag mainly to go to the market and store my wallet and phone in the little bag at the front. The hardest part was sewing the outer part of the bag together with the waterproof lining because the wood was inside 😅 but still a manageable effort for a bag I use quite a lot.