[asid61]

For actual stuff without dimensions, Blender/ Maya FTW. 3D printers are great for this.
But if it touches the robot, don't use Blender of Maya if it's 3D. If it's like a 2D plate, maybe, but you would need to scale it to approximately the corrent size. 3D I would probably avoid just because it would be tricky to mount anything 3D.

Anything structural should be Inventor or Sketchup.