I was just looking at the ram mounts. If you don't want it mounted on the chair an air desk might work for you. I use one for my laptop and it also holds my kindle. They spin out of the way easily.
I just have my laptop sitting on a large tray that came with my wheelchair. I sit it on a cooling tray to keep the battery cool. The tray is wide enough for me to put a mouse pad on the side as well.