I think you are right about injection moulding not being suitable, because you are trying to achieve one large thin feature. I suppose rotational moulding could work instead. Stretch-blow would require the material to be handled in a state that allowed it to be stretched before blow formed, which would limit your material choices.
Although Dip Moulding, as you described above, is very cheap already, as there are no pressures involved, and very few moving parts (comparatively). I cant think of anything that could be quicker either, but maybe someone else knows something.
Is there any particular reason why you are avoiding dip moulding?