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
Easy to print and comes out great. Adjust the infill percentage to suit your needs. I used grid at a percentage I can't remember. Just be sure to use enough infill to support the top surface of the larger piece so that it prints smoothly.