How to safely insulate the Power PCB of a display computer monitor?
You see that thick white line on the power supply board? Everything to the right of it is to be assumed to be at mains potential, that line marks the safety isolation barrier. 

You cannot pot a HEATSINK! Think about it, the whole point of the thing is to have surface for air to flow over to remove the heat.

I would note that each of those board has a perfectly good set of mounting holes, get some ally, some standoffs and make up a case (With vent holes too small to admit a finger), job done. You will want to make sure the metal case is connected to the power supply earth connection (The green/yellow wire bottom right of the power board).