I use @Strankler's adapter to stack two Ikea lack tables:
https://www.printables.com/model/11146-ikea-lack-stack-adapter

But I use my own 4cm leg extensions with 3rd lack desk (for getting full Prusa MINI+ 18cm print height with Lack table above), so cannot use @Strankler's adapter to stack that:
https://www.printables.com/model/157761-screwless-sparingly-ikea-lack-leg-extensions-in-4c

I created a stack adapter that looked similar to @Strankler's wrt bottom table. And I wanted to reduce material as much as possible for connecting with 4cm leg adapter on top. This is the first time that I created two parts that need to be used together for the job. The two parts have no overhanging parts, so 3Dprinting without support has no issues. My 3 Lack table construction at home shown in 2nd photo (hosting my Prusa MINI+ 3Dprinter between 2nd and 3rd table) is a complete “no glue, no screw” construction.

The Prusa MINI+ .gcode files print 4 of both models needed to stack 4 feet with 4cm leg extensions.

Attached Freecad files allow for changes if needed.

Related forum posting:
https://forum.prusa3d.com/forum/general-discussion-announcements-and-releases/ikea-lack-table-raising/#post-602261