Mounting 20x80 to universal plate with wheels

I'm having a bit of a problem. I want to mount a universal plate with wheels on one side to a piece of 20x80, but the screw heads that hold the wheels on cause a gap:



If I just screw the plate into T-nuts, it flexes downward. How is one supposed to mount a plate in this situation? Spacers? Some other bracket? Mill out a pocket for the screw heads? Different screw for mounting the wheel that sits flush? I bought these bits many years ago, so there might've been changes.

Thanks!