I wanted to be able to stack two Ender 3 S1 printer on top of each other. I do not have the second printer yet but the space below has more than enough clearance for another printer with the spool on top.

There's only one file. When two are printed, flip one and align with the first.

One side has 3.5mm holes and the other has 4mm holes. I heat sunk 3mm knuckled nuts then used M3 screws to hold it together. If you don't want to use knuckled nuts you could easily use a regular nut.