Instead of measuring and printing a basic rectangle to act as a spacer each time a drawer was reorganized, I created this adjustable spacer. The teeth are spaced ~3mm and the clip can slide to the appropriate position and will stay in that position.

FreeCAD file of the clip has been included to adjust the length and thickness of the clip to your needs. I suggest increasing the thickness of the clip as the length increases, and vice versa for decreasing length. Its personal preference how stiff you like the clip.

Length: Black circle in the screenshot.

Thickness: Blue circles in the screenshot.