Do not increase stifness of the blade holder disc. Ideally, it should not have any stiffness (i.e. no transmission of blade bending moment to the head through a hinge) to avoid dissymetry of lift. Can be done using polypropylene sheet sandwiched between wood except at hinge location (Jim Baxter design) or more simply by using a glass fiber plate (not more than 2mm thickness) if you add enough tip weight to the blades.