The one many of us have that bolted right up on the back seat location was made by Show Chrome (Big Bike Parts). Long discontinued. You might be able to find one used at Pinwall Cycle (large cycle salvage in OH, and on Ebay) or at other cycle salvage yards. I do not know the original part number.
It was called a Solo Rack.
Some US made ones still exist, but do not bolt up on the Valk.
Now I seem to remember an old thread on here where someone figured a way to modify one to fit the mount points on the Valk, but it was some time ago. This is the current Show Chrome solo rack that is not made for Valk, but I believe is the one someone figured a way to mount on a Valk.
https://www.amazon.com/Show-Chrome-Accessories-63-131-Solo/dp/B001AWFRYQPerhaps a good researcher might find the old post on this.