Hello,
now probably a bit late, but maybe it helps others:
Ikea assumes that "filler strips" to fill small gaps are made from Ikea "side panels." That actually works quite well, but the side panels are not as thick (about 15 mm) as the fronts (about 19 mm), but that doesn't bother me. The side panels are available in every front color.
So cut precisely (always with a guide rail for the circular saw and keep in mind that one side tears out a bit and the other is nice and clean, so do tests!). I painted the cut edge with matte varnish RAL 9001 because I have fronts from Bodbyn light ivory. The color fits perfectly. Then glued the filler strips, for example, with mounting adhesive.
I am very satisfied with it.