Failing that, a door with one edge rested on a radiator and the other a stack of cardboard boxes.
I work from a proper ergonomic office desk now but it beat many other alternatives including a dining table and various purpose-made 'home office' desks.
(Accepting I got lucky with the height of the radiator).
In the same vein I worked off a folding table for a long time. It was cheap, large, and surprisingly sturdy. If I needed the room it took up back I just folded it away.
