I had the same issue with finding a bracket that fits across the windscreen mount posts, to mount by GPS, so after removing the screen to give the inside a good clean, I could see how I could construct something to do the job.
I used angle brackets on each end, bent them to fit over the posts, used a 25mm hole saw to cut a section at the bottom of the angles so it would sit low without interfering with windscreen operation. Used a section of 22mm steel tube and held together with a 6mm section of threaded rod with Nyloc nuts on each end. I spot welded a couple of small welds to prevent any rotation. Sprayed with Matt Black paint and bolted to posts with a couple of 5mm hex bolts and Nyloc nuts. Works a treat.