I have used footplates on a number of my projects. These were all made by hand and tended to be a bit tedious to reconfigure for different sizes. So I decided to create a parametric version for hexagonally packed openings which I uploaded here. It was a very simple job to adapt this to a diamond pattern which is presented here.
The OpenSCAD model does not include any safeguards for restricting the holes sizes and corner radii to be plausible so if you find the grid has dissappeared or your rim is gone, check the relative sizes of your Outer and Inner radius and the holes sizes.
You can find the hexagonal-circle pattern footplate here: Parametric Footplate
and the square grid version here:Parametric Footplate - Square Grid Pattern
I hope you find these useful