Your method of using framework and woven roving appears good, and it's great to see 'out of the box' thinking. Did you consider using foam panels to get to the shape? the foam spray is very expensive (I know from experience
), and the foam sheet is so light it can self-support. Foam panels (sold as builder's insulation) isn't too expensive, and can be glued on with your foam after the paper and/or foil sheet is removed (that process is very smelly for some reason!)