The thing is, the French government didn't really want to sponsor a massive settler colony. It feared ( irrationally) that France would be "depopulated" if large numbers of settlers were sent, and (more rationally) that this would alienate the local Indian tribes and damage the fur trade, which provided the only real export commodity.
So to have a more populous New France you need the French government to change its whole mentality. The low population wasn't just because not many wanted to go to Canada, but also because the government was basically OK with that. It wanted to have just enough people there for the colony to be able to defend itself, and that was it.
Could you engineer some sort of population boom, leading to metropolitan France becoming over-populated and the government encouraging overseas settlement as a way of alleviating this?