If you want a smaller one for your MacBook or, the way I use my store-bought unit most, as a bedside mousepad, they're cheap or you can make your own. When it comes project time, though, I'd rather not mar my lap desk, my desk-desk, or my kitchen table, and I can always use more surface area.
In the way that projects beget projects, resurfacing some thrift-store furniture with contact paper inspired me to put together this beauty:
Refined, I know. The main components were an old bulletin board and some black packing foam, joined with contact cement. The frame was already broken, but it happens to be useful having two sides open, and the piece from the bottom makes a passable straight-edge. Scraps of contact paper provide some smooth surface area while the cork remains mostly exposed for cutting, pinning, or anything messy. I keep an old gift card and some thumbtacks on mine, but you could attach any kind of hardware, including screwing on a zippered pouch, pencil case, or divided storage bin to keep small tools, pencils, or a razor knife handy.
Here's the underside:
The foam came with some computer equipment and works great, being just pliant enough to be comfortable in the lap while providing a stable foundation.
If you don't have these exact materials on hand, consider what you do have. To me, DIY is all about re-purposing and improvisation, not drawing up a blueprint and driving to Big Box Hardware or Ikea. This beast has come in handy for half a dozen projects, plus serving standard lap desk duty when its petite, prettier counterpart is in another room.