Summary

Included SCAD file if you wanna customize it using the customizer- choose the number of cells, size of panel and magnets, thickness of panel, size of bolts, tolerances, etc.

Updated to include option for cylindrical magnets in the customizer, and to toggle bolt holes- just set the option to 0 to disable it. Anything above 0 will turn it on.

The example file is a 100mm x 100mm tillable panel of a magnetic pegboard using 25 4x4x4mm cube magnets, spaced equidistantly 20mm apart at center of the magnets.
Has mounting holes using four M3 countersunk bolts
The M3 holes are 3.25mm for tolerance reasons, and the holes for the magnets are 4.25mm

There's also a 6"x6" example with magnets set 1" apart.

If you want a 6" x 6" panel with magnets spaced 1" apart, set these values:
Panel Size = 152.4
Column Row Count = 6

You can usually get these magnets as a set of 216, and comes in a cube of 6x6x6 magnets, I think the brand named toy version were bucky cubes, but you can get these magnets off magnet warehouse websites as well