You could adjust the size of the border overlay by changing it's anchor points. Something like:
Code:
f.Texture:SetPoint("TOPLEFT", -4, 4)
f.Texture:SetPoint("BOTTOMRIGHT", 7, -6)
You will have to adjust the overall size of the button.
LibDBIcon does a much better job of making MM buttons look/work like the Blizz. ones but ...