Inspired by the way the Prusa Lack Enclosure aligns the top of the enclosure to the base table, I created a similar setup so that I could securely stack a third lack below my enclosure without screwing it together.

Simply screw four top piece to the top corners of your base table, and then screw a leg piece to the bottom of each of the legs of the next table up.  The cone on the top piece will register with the void in the piece on each leg, and keep the tables from sliding apart.  There's enough of an offset between the cone and the void that the legs don't have to align perfectly (since that's a bit to much to ask of self-assembled $10 Lack tables).

I printed my set out of PETG with a 20% infill, and they've worked well for 8+ months so far.