You will need a Drawing Program if you want to have your own "Custom Chassis"!!! be creative, dont follow the paths of other aftermarket companys. You can do it!! A good drawing program is Inventor(Creatative drawing with 3D imaging) or you can use Mastercam(CNC program)
My friend and I desinged our own chassis for the TC3. Intstead of "following" the norm we decided to have the batteries in a 4x 2 configuration or a 3x3. The car handels like a dream, and the balance is perfect.
The only draw back is making the chassis. It costs alot and takes a long time setting up the milling and lathe machines.