The commercially available mounts that you have seen are fairly expensive but very high quality. The same stuff they use in police vehicles.
I went to a Boating supply store(West Marine) and bought some Ramline mounting hardware and then fabricated a desktop from baltic birch plywood to my own design/needs.
This works well,I have made several for different vehicles and computers. The Ramline hardware is high quality and stays locked in the position I put it.The downside is that it takes some time and imagination and crafting skill to get it right,but I fabricate stuff a lot anyway so I actually enjoy doing these kinds of things
I have also seen various desks that mount in the passenger seat that would work and Adrian Van and Weathershield both make desks. Anything pre-made would be expensive though.
Good Luck
Ed
