Take a look at the photo of my bike below. I have a leather fork bag that mounts on the top of the upper triple tree. It's about 10" wide x 4.5" high x 3.5" deep. It has a pied of "L" aluminum angle on back that bolts to the two screws on top of the tree. Fairly stiff, thick leather, with a dzus fastener for closure. I use it for a cell phone, spare light bulb, allen wrenches, a rag. I can also cram in a small water bottle.
The problem is I don't know who makes them. It's not a Harley part, although I got it at a HD dealer in NC. I have seen them on ebay from time to time so you might look there. It's been some years but I think it was about $100, a bit steep for a tool bag, but this one was the only one around that attached this way. I had a soft, strap-on tool bag before that wasn't nearly as functional.