I think that easiest way to solve the problem of rotating of plate P<sub>2</sub> will be using a mechanism like this:

![hinge mechanism][1]

To use it, join plate P<sub>1</sub> to an additional plate with length of 10 cm (height and width depends on the dimensions of mechanism) like this:

![sketch of additional plate][2]

After join of "unknown element" with P1 and after placing P2 on "unknown element", construction will look like this:

![sketch of linkage between plates][3]

I still need to find way to prevent plate P<sub>2</sub> from falling, I don't think that screws will be enough to attach this mechanism to the plates.


  [1]: https://i.sstatic.net/0uTUr.jpg
  [2]: https://i.sstatic.net/HO1dD.png
  [3]: https://i.sstatic.net/znTFG.png