Hi,
While I personally don't like the idea of people being employed for "cash", I think Greg has the right idea.

Over here in NZ, you can hire a person on a "casual" basis, as in "when needed", they are your employee, but you only pay them for the hours they work (obviously).

As a casual employee, you have no sick leave, no holiday pay,
but you do have accident cover under your employers contract, that is a given.

Some people I know work 2-3 jobs like this and strangely enough, they are actually happy with it.
Not my cup of tea, but different strokes for different folks, I guess.