We've since tried a couple of things. First was a "C" table we had from our old RV. Its about 15x17 and since it only has the one leg on one side it makes it easy for the passenger to get out of the seat while moving to run down the latest rattle. The one we have it too tall as it blocks view of the radio so the final solution might be a custom build.
We also tried a lightweight table I built for another purpose but it ended up being too big and with four legs made it difficult to move around. I did locate an ottoman on Amazon that has a reversible top between a tray and cushion but we've decided that the "C" table provides better access.
On the side of the drivers area I made two wedge shaped pieces of wood and mounted a piece of plexiglass between them at the angle between the window edge and the dash. Its about 16" long and provides a surface for attaching suction mounts for a GPS, TPMS, and wireless toad brake. By using it I can still open the window.