Inspired by @MCollins_25719 print sheet holder https://www.printables.com/model/19318-steel-sheet-holder-for-prusa-i3-under-table-mount 

One set of holders are used as a spacer (gray prints) for the screws that would otherwise be in the way of the outermost sheet. It fits an Ikea Kallax shelf unit.

For my setup with 3 slots and one spacer i used M3 x 35mm wood screws.