It depends on how nice you want it to look. If going all-out, I would probably make my own pillow cover, put two pillow forms inside, and run an elastic strap between them. This would require making a custom pillow cover with two bound holes for the elastic strap. If I didn't care how it looked, I would probably just find a used pillow of approximately the right height and firmness and then ask my husband what glue product to use. Or, instead of glue, you could use heavy-duty sticky velcro between the pillow and the lap desk. Big box hardware stores usually have the heavy-duty velcro.
Probably one thing you want to do before deciding on a pillow is sit in your favorite spot with the lap desk on your lap and have someone measure the distance between your elbows and the lap desk. That would be the height you want for the pillow, I think. I'm not an expert on ergonomics, but I know that for typing and sewing machine work, you want the surface at about the same height as your elbows. This puts the least stress on your wrists, neck and back. I think you would want a rather firm pillow, as you don't want your work sinking into the pillow; you want it to provide support. Instead of a pillow, you could experiment with other things. I am thinking a small pet bed of the correct height (a pet bed that has both a top and a bottom) could be stuffed with styrofoam and velcro'd to the lap desk. If you don't mind the weight, you could stack old magazines or telephone books to the correct height, secure them with twine, then cover with heavy batting and a pretty fabric. Or maybe just cut and stack styrofoam pieces to the correct height, tape around the styrofoam to hold it in place (I think duct tape works on styrofoam), then cover with a couple of layers of batting and some pretty fabric and velcro to the lap board. With styrofoam, you could even round the edges a bit.