Isolating main power traces on backside of the PCB
First of all, having the PCB properly manufactured would include solder resist all over the back side. Nowadays you can get reasonably good PCBs for a few dollars, if at all possible avoid the perfoboard and use a "real" PCB. If that is not possible, painting or conformal coating is the way to go. Make sure to use proper, non conductive paint.

That said, some sort of plastic sheet is commonly used in commercial products. I am not sure of the exact material, it is quite thin, maybe one fourth of mm, clear and reasonably flexible.

It is also of prime importance to screw the PCB to the case using an appropriate number of mounting holes, and spacers. If the PCB is rigidly connected to the case you can probably skip the plastic, and even the paint, but safety is reduced of course.

For a small, homebrew project a perfo board well fixed to the case is more than enough. If you need to bring this thing around, on stages, where people can possibly touch/spill something on it, please use a real PCB, or paint it with the appropriate material.

Be safe out there! You don't get to touch the mains too many times.