Cheap solution for underside of barn overhang (soffit, siding, something else?)
At first I was thinking it was just some exposed rafter ends, but you have an entire roof there!  Short of using a netting like you have already done, I cannot think of anything inexpensive that will do the job any better. You have the option of putting some exterior sheathing such as 3/8" exterior glue plywood or maybe some T111 siding panels. They are not really cheap, but would be a permanent solution. Since you have a metal roof and don't want to trap air and moisture in there, plastic sheeting etc would not be a good choice. Seeing it, I would have recommended bird netting, but you already thought of that.