Heated bed has a bulge?

I have read the FAQ and read the thread about fixing this issue by drilling out the holes and have done this but it has failed to work. So my corners are lower than the middle, putting the glass on has helped much either. Wondering if I drill a hole in the middle of the aluminum plate and epoxy a flat screw to the bottom of the heat bed to pull the bulge down would this work or would the epoxy and screw have a bad effect on the heated bed? any other IDEAS how I can fix this?

I had the same issues with my buildplate.
The final solution was :

  1. remove all scews that fix the heatbed PCB (the PCB bends because it stretches under heat and is fixed by the screws)
  2. turn the PCB round (traces facing upwards)
  3. put a glass plate on top (with thermal compound)
  4. fix the headbed “sandwich” to the Aluminium plate with document clips on four sides.

Additionally i put a layer of cork between heater pcb and aluminum to insulate the lower side of the heatbed.

cheers,

Christian