EX: Bike couriers are a good example of relatively high pay and often minimal documentation.
Again, I don't think you could knock the ball out of the park or allude a motivated state agency if you were a terrorist or something; but if you were in a similar position to the subject of the article you could probably avoid capture and maintain a semblance of normalcy.
You could also become a loan shark, which can be lucrative, but your illegal status would give your customers leverage against you, which is something you really don't want if this is your retirement savings...
Becoming a loan shark is a possibility, I suppose, but puts someone in direct competition with professional criminals and creates a whole new set of existential and legal problems.
One low-risk strategy I've heard of being used by cash-savvy people is purchasing food stamps at a discount to their face value. Again, pretty much any kind of black market activity comes with legal risks.
I was replying to the idea of $40k (cash, untaxed) as not much money. It corresponds to the take home of a middle class income in much of the US.