I, too, ordered the X-carve (Shapeoko 2, in my case, actually) without a wasteboard, and made my own. It’s actually not all that complicated, and while the wasteboard does supply some stiffening to the CNC, it can move and work without one.
I’d suggest that you go to your local wood supply store, and purchase an appropriately-sized piece of MDF, and custom cut your own wasteboard. Inventables posts the designs for their parts, as they are an open-source company, and the wasteboard design can be found here: GrabCAD plans.