Pretty much what you described, minus the floating: a laptop riser plus a separate keyboard and mouse.
The numbers from OSHA's computer workstation guide: top of the screen at or slightly below eye level, center of the screen about 15-20 degrees below eye level, about an arm's length away. Keyboard and mouse close to your body at or just below elbow height, wrists straight. OSHA's purchasing guide says laptop keyboards aren't suited to long typing because seeing the screen properly puts the keyboard too far away, so it recommends an external keyboard for any laptop used as a main machine.
Cheap setup: a folding stand (or a stack of books), any wireless keyboard and mouse. Better: dock it to an external monitor and use the laptop screen as the second display. On the couch or a plane, just change position often and keep the sessions short.
