I have rigged a Toyota wind up spare tye carrier under the rear of the Heaslip. With the existing spare mount on the front that gives me 2... and as it uses the same wheels as the Jack I can effectively carry 3 if I need to.
The only down side to the rear spare is that it reduces the departure angle, but probably no more of a concern than a rear water tank, and shouldn't be a problem in places like the Gibb.