I have a nook in my new bedroom. What would be the easiest way to turn it into a desk area without buying a desk?

The space is 40 inches wide, 20 inches deep. I’d like to use it as a desk space, but would prefer not to buy a desk – I can’t really afford one, and I don’t feel like owning one (I’m sort of a minimalist and I move a lot, and I don’t like having a bunch of stuff to lug around).

I imagine this would involve a wooden board of some sort? I don’t have any wood-cutting equipment on hand, could I go to Home Depot and get a wooden board cut to my specs? How would I fit it in the nook? Maybe rest it on some nails that I’d hammer into the wall?

How would you do this?