Dix_Daim's design for these stops is functional, but didn't fit my bed frame exactly. The plugs were too large and the slot for the slatted frame was too thin (the wood did fit with use of violence, but that's not exactly a "fit", and it broke apart within a minute from internal pressure).

I increased the slot's size so that it's little less than centimeter larger on the x-axis, fitting 5.7cm wide beams now; fattened the wall's thickness for more sturdiness and reduced the plugs' size to fit my metal bedframe.

I have no idea how Dix_Daim printed this without supports - I recommend normal supports and vertical printing.