I use a large coffee table book. I think it's an atlas about an inch thick. You can pick one up at a bookstore for about $10 in the bargain area. When not in use, you can look at all the pretty pictures or set it on the coffee table. 😎
I saw something very interesting on a webpage a few weeks ago that I wanted to try. Id have to do a search to find it again. I believe it was on a webpage called...hackikea....something like that. Basically they took some kind of stool or table and cut the legs off and it made a perfect laptop lap desk. Let me see if I can find it....