If painted on one side only, cardboard will curl. And in height by 2 feet. Two flats that are joined at a corner will stand by themselves, as will an open book. It can be a inside or an outside corner. Three flats joined in a 'Z' configuration will also be self supporting.
Jacks are simple to make and use, and use, and use.
When dry the upright is fastened either directly to the back or the edge of a flat with screws. Jacks can also be fastened to the back of flats with hinges.