After reading this topic I had a look round, bought a German made mic stand iPad holder and a cheap mic stand and had a similar setup in no time. Fortunately so far I haven't needed the counterbalance. I did have to make a small alteration to the holder. It would swivel downwards 90 degrees but a small rod in a curved slot stopped it tilting backwards. A quick hacksaw job shortened the rod so I could tilt it in both directions. I would never have thought of the idea without your initial description.
£30 for holder, £14 for stand. With carriage costs, total around £50. Much less that the nearest commercial product to do a similar job.
John (the Grimbarian)