Yep, the "People's Temple" succeeded in securing an area of 15 km˛, not that bad. Though they technically didn't found a State, they negotiated favourable conditions with the government with regard to customs and emigration. Unfortunately, a "communist prison camp" usually doesn't work out very well.
As for the legal / illegal foundation of a State - the scholars of the International Law apparently agree that in most cases, any illegalities involved in the emergence of a new State are healed by the mere fact of its existence.
In Oppenheim-Lauterpacht: International Law, vol. 1 , 8th ed. (1955), p. 544, we read:
„The acquisition of territory by an existing State and member of the international community must not be confused, first, with the foundation of a new State [...]. The formation of a new State is [...] a matter of fact and not of law. It is through recognition, which is a matter of law, that such new State becomes a subject of International Law. As soon as recognition is given, the new State’s territory is recognized as the territory of a subject of international law, and it matters not how this territory was acquired before recognition.“
Alike, R. Y. Jennings: The Acquisition of Territory in International Law (Manchester University Press, 1963), p. 8 :
„For transfers of territory between existing States the law lays down a series of modes through which alone a valid title to the sovereignty may be passed from one to another; but for a territorial change coincident with the birth of a new State the law apparently not only fails to provide any modes of transfer but appears to be actually indifferent as to how the acquisition is accomplished.“
This sounds encouraging to me...