I modded the leg connectors from the Prusa printer enclosure parts to stack an additional table on the bottom without walls. Makes a nice stable table stacking coupler for other purposes as well. I attach them with #10 x 3/4" wood screws and a little glue to keep them from rotating after they are attached. Works great.

Check out these other things for IKEA Lack table...

Table leg levelers: https://www.thingiverse.com/thing:3495732 Leg stiffeners: https://www.thingiverse.com/thing:3490946