Here are directions on making a lap desk. Wood base is available at Home Depot for $6.
http://www.abeautifulmess.com/2015/0...-required.html
There are quite a few options out there that aren't terribly expensive - less than the Lap App. Search "raised lap desk". Even at Walmart or Amazon, you'll see several choices. On pinterest there is one made from a cupboard door that looks like it could be easily adapted to the size you need, whether you start with a cupboard door or a piece of wood. I like it as it has an raised edge to keep supplies from rolling away.