STACK-A-LACK

The STACK-A-LACK is an adapter to stack IKEA LACK tables on top of each other. No screwing, no glueing, just stacking. It wasn't designed to be very sturdy, but it should keep the top table from sliding off.
There is also the DOUBLESTACK-A-LACK connector that can be used to combine two LACK stacks.
If you want to stack two LACKs, print the STACK-A-LACK plate.
If you want to rack two stacks (four LACKs), print the RACK-A-STACK-A-LACK plate if your bed is large enough, or both the STACK-A-LACK and the DOUBLESTACK-A-LACK plates.
The RACK-A-STACK-A-LACK plate contains the adapters for all four corner and the two middle connectors.
Print Settings
Printer Brand:
RepRap

Printer:
Original Prusa i3 MK2

Rafts:
No

Supports:
No

Resolution:
0.2

Infill:
100%

Notes:
Considering how thin the walls are, it doesn't really matter how much infill you use. I'm printing them solid, just to get a bit more strength and a nicer outer perimeter. My shell thickness is set to 1,2mm, and that's also how thick the walls are.
Post-Printing
Assembly
Slide a STACK-A-LACK over each leg of the upper table, with the closed square at the top and the half square at the bottom and on the outside of the leg.
Place the upper table on the lower table and make sure that the STACK-A-LACK slides over the top of the lower table.
Enjoy your LACK stack.