I'm not sure any thickness is appropriate with a "rear only" solid set-up. IMveryHO, if you use a very rigid rear only solid mount, then you have forced the weak link to go elsewhere (perhaps the aluminum diff cover?). And this goes for a solid front only mount too. My thoughts on this is either they both be solid or both be flexible, but not mixed. If one side is allowed to flex (poly or otherwise), then movement allowed by the flex mount is passed to the solid mount. With enough movement, frequency, and time, the solid mount will fail (327 knows) or the material around it will (sheet metal). Is there a material that will not fatique under repeated stesses and flexing that can be used as a solid mount? Perhaps part of the wing of a B52?.