You can use anything as a mold as long as
1) it does not exceed the forming area: 9x12x6.3 inches
2) it won't collapse under the vacuum pressure
3) it can absorb and dissipate the heat of the hot plastic coming into contact with it
To explain no. 3, imagine hot plastic coming into contact with a 3D printed mold. If it's a thin sheet of plastic, it will transfer a small amount of heat to the mold, and the mold temperature rises by a few degrees but disspitates it relatively quickly. But if it's a thicker sheet of plastic, the mold will absorb more heat. It may heat up so much that it begins to soften before it's had the chance to some of that heat.