Step 7: Other Thoughts
Update: view Supafly's comment below for the "leg-nail" mod. Supposedly it works great, and it seems the best/easiest way to assure no slipping. Thanks supafly! If you can't find his comment below, then here's a basic description: Drill holes in the bottom leg starting near the joint and going out along the leg. When you assemble the leg and open the joint, you can put a nail in one of the holes and it'll stop the upper leg from sinking. You can move the nail from one hole to another to accommodate different angles, and you can remove the nail to allow the whole thing to collapse.
The length of the legs will be determined by your stomach size, arm length, and how comfortable you want to be. I make it as short as will fit over my gut. If you wanted to get creative, you could cut out the portion of the lip/tray that touches your stomach, as long as there is enough lip on either side to hold the laptop from sliding off. A small indent like this may give you an inch or so (till the laptop is resting on your stomach), which may make a difference in comfort.
If you make one, please post up and let us know how long it took, how much it cost, and what it's like to use it. If you have suggestions, post them up. Good luck!