Serpent have introduced a roll center spacer for the Spyder SDX4. Made from 2mm aluminum hard coated, the spacer fits below the differential into the chassis.  You can also put 2 on top of eachother to create 4mm spacer. The design is made such way that the 2 spacers will also lock in position. Depending on the plates use you may need to also space out the rear stiffener. Increasing the roll center will move the differential higher as well, creating more grip and steering, but less easy to drive.

See: Serpent