I don't so much have a desk as a "cubby house" for my laptop that I put together with a cheap plastic table, plastic container and a couple of door-stop clips. Total cost ~$50 all up. It's more stable than it looks. I have a red screen over the laptop, but this limits extraneous light impact to others, keeps the dew away and puts the laptop at a comfortable height.
http://i1264.photobucket.com/albums/...t/29eef15e.jpg
http://i1264.photobucket.com/albums/...t/8880e3ed.jpg
I'm limited by what fits in the boot of my car, so large desks, though attractive, are a no-go for me.