I have two wooden plates with the same dimensions (5 cm thick, 50 cm long and 35 cm wide).

The distance between plates must be at least 10 cm. Plate P₁ is fixed but P₂ rotates about the y-axis. Angle alpha can go from 0 degrees (when the plates are parallel) to at least 90 degrees.

How can I realize this design in practice? Which mechanical elements should I use? It must be strong, there will be load of max. 30 kg hanging on plate P₂. There must be some mechanism that will connect these two plates and enable P₂ to rotate but when one sets a desired angle between plates, the mechanism must hold the angle constant.