A voluntary society cannot be designed at all. It will be emergent. When a critical mass of people realize that the rules we tell children to live by (namely don't hurt people, don't mess with their stuff, and keep your promises) should be applied across the board, and that no other general rules are necessary, then such a society will form.

There can be no formula for dealing with people in need. As soon as such a formula is known, most of the marginally needy and some of the non-needy attempt to game the system. Subsidizing poverty creates more poverty. The best way to deal with those in need is on an individual case-by-case basis. It's too important of a problem to be left to monopolists. Concrete answers are wrong answers.