You should try the "flexing" idea. I know it sounds really scary, but this is how I do it -
1. Put the board down where it needs to be
2. Apply pressure to all four corners of the keyboard (I have a tenkeyless, but I'm sure it would work just as well for a full size).
Basically just picture you're "flattening" an uneven sheet of foil, perhaps applying more pressure to the high corner. Also, I am speculating that tenkeyless models may be more prone to this uneveness. When I was first playing around with the "flexing" idea, I was twisting the keyboard, as in, I had the keyboard in both hands and was manually flexing it like how a pissed off German kid would after losing a game of Dota or what have you.
This produces way too much torque so go with the flattening method. It works and my Filco has not sustained any damage.
*Edit - Apply pressure to the board using your thumb and index finger of both hands to the corners of the keyboard