Most brackets for stacking IKEA Lack tables either require hardware or are not secure against forces in all directions.  Typically they will prevent the leg of the upper table from sliding inward on the top of the lower table but, for forces trying to slide the leg outward, they must transfer the force up through the leg, across the tabletop, and down other legs resulting in significantly less resistance to these forces.

This two piece bracket resists forces in any horizontal direction for each leg.  In addition it prints without supports and requires no hardware.

You will need four of each of the top and bottom pieces.  Put the top piece on the leg of the top table upside-down from the way it was printed.  Put the bottom piece on the leg of the bottom table in the same orientation that it was printed and slide it up on the leg until it engages and snaps into place with the top piece.  I used PETG but PLA might also work.