Buy a foam beer cooler at your local food store.....
Seriously, the design is amazingly tricky if you are looking for efficiency and utility. Vacuum insulation is the most efficient, which means you need to create a vacuum tight structure and evacuate it. The most efficient machines use an inner chamber that hangs inside an outer chamber and only touches at the lip to minimize conduction of heat.
Remember that every pound of metal on the inside will take liquid nitrogen to cool it just as if it were a payload item, so you need to keep the inside of the machine light. Use a stainless steel, as its heat transfer is less.
You can make a processor with foam insulation, but it will use a lot of nitrogen, and be very expensive to run. You won't be able to apply heat to the chamber because the foam will not take it. The foam will break down over a number of cycles, and the machine will become progressively more and more inefficient.
Some processors use a solid insulation such as vermiculite. It too will be very inefficient, but you will be able to use heat in the machine.
Air circulation within the machine will be important. Corners become hot spots because of poor air circulation and increased heat infiltration. Getting the nitrogen into the chamber is also pretty hard. Some units use spraybars. We've found that they can spray liquid nitrogen right on a part and crack it. We find it best to use a heat exchanger that cools the air in the chamber which cools the part.
