Dimple Die bending the part

I find when using dimple dies, they often bend/curve the part that I'm working on.

Here's a custom dead pedal I just made, you can see the curve after dimpling the holes.

Any tips on how to prevent this and keep it nice and flat?

Dimples put stress into the material and also suffer from a 'spring back' as part of the deformation is elestic.

A common problem is not fully tightening the die down, as this will reduce the affect, but either way it's not uncommon for some finishing to be required to remove the distortion.