It came from Amazon, but isn't there now (or on Ebay or Ali) that I can see. It was sold as a 5.5L Scooter Handlebar bag. Two Velcro straps at the top go over the handlebars and one at the bottom to go around the stem. Equally, it attaches to the D11's handlebars/stem, or saddle rails/battery, or the rear rack. It has a hand strap and supplied shoulder strap too for carrying which I found made that type of bag more versatile than the usual bicycle trunk bag.Do you have a link to the trunk bag please? Does it attach to the seatpost natively or did you have to figure out how to attach it?
There appear to be many similar/better bags now, especially on Ali.