I would go for a metal plate rather than plastic as a metal plate can also be used for cooking such as baking with a reflector oven. The snowpeak titanium plates are excellent being corrosion resistant and extremely durable. The additional cost over a plastic plate may well be offset by the additional flexibility and long term use it will provide. Traditional bake ware is off course available at your local superstore but these tend to be over sized and heavy for backpacking.

You can also make some of your own gear as well to save money.

http://www.kayak2go.com/reflectoroven.PDF