Have you investigated current incubator designs to establish their critical operating components and key performance criteria? If not, I would recommend you search for information for the existing equipment. Many times the cost of our medical equipment is more a factor of the cost of the approval processes we in the developed nations demand to insure safety rather than a result of the basic design requirements and elements requred for the equipment's primary function. Also, if you want see an extreme case of alternative incubator designs then also search for those units used by farmers and naturalist for eggs and infant animals and birds. This way you will learn the two opposite extremes of the designs and be more able to see where cross integation might lead you in your project.