If, for the purpose of double-sided coating, you want to mechanically fix the PCB so as to prevent it from falling out, the central fixation combined with the ball fixing is the right choice. Both elements are installed facing each other, while each one has two supporting brackets above and below the PCB.
The ball fixing contains a spring-loaded ball. When inserted into the ball fixing, the PCB is slid in towards the spring-loaded ball until it can slide into the central fixation on the opposite side. Once they have been properly set, both elements will securely fix the PCB in position. In this way, the PCB cannot fall out at the top or bottom.