You know those lapdesks you buy in stores for $40+? As Instructables user kandrejk discovered, they're basically a cushion glued to a cutting board. You can make one for yourself out of items you probably already have or could purchase for under $10.
Kandrejk's instructions suggests actually making the cushion yourself — which you can definitely do if you want to break out the sewing kit — but chances are you can find a cutting board and a standard cushion (e.g. for a chair or bench) that are about the same size. Glue them together and you've got a lapdesk. If you want the fully customisable experience, however, hit up the Intructables post for the complete how-to.
DIY Basic Lapdesk [Instructables]