You would need a pretty big heat spreader anyway to avoid thermal shock. I would advise using at least 5mm thick plate for the spreader. In terms of evaporator design I would look at a chamber design that is about 50mm^3 at least. Aim at building in vertical spill over chambers inside to trap liquid refrigerant but allow the escape of vapour. If you plan to use a TEV then it will help tremendously.
Here is a source of excellent info on TEV's and evaporator design and performance. You can find an excellent piece of software for design of refigeration and simulate the effect
here