There’s a 4x2 file on Cutrocket:
which could be adapted (see below)
and a guide for the 4x4
The problem of course is that it’s not really possible to make new slats in a straight-forward fashion — they don’t quite fit, unless you cut them on a diagonal, which is a pain:
Or, of course, one could tile or rotate if the boards are the right length, or if one cut them to length w/ the second operation.
I believe @Radiation worked up a file for cutting strips which are a bit short — check in at support.
We don’t sell them, MDF is heavy, and the square corners are fragile and breakage rates for such are high in our experience.
I don’t know of any 3rd party sellers, but it should be possible to use:
to have someone make them w/ a table saw and a drill press — or just pull a filler strip in reasonable condition and use it as a template.