Several mass customization toolkits for consumer products are compared amongst each other from a design perspective. The customized products are all produced with rapid manufacturing technologies and in all cases the shape of the product is the main variable attribute which can be customized. The aim is to determine criteria that define the generic process of this co-creation approach. Comparison is used as a method since it focuses on understanding the customization tools and the process. The first part of the paper is concerned with the theoretical approach and the comparison of five toolkits. The second part deals more in-depth with the design of solution spaces which focus primarily on customization beyond aesthetics and it also deals with the design of validation tools. The expected outcome is a generic process of co-creation which highlights valuable aspects and can be used for the future development of mass customization toolkits.