Lack is made out of super cheap particle board. Is it stable enough to sit the 3D printer on? I would assume it would add some errors to the printing process.
The downside about the IKEA particleboard over hollow cardboard core is more about sound and resonance imo. It can act as a speaker for printer vibrations and amplify that sound in the same room or to the floor below it.
A popular “mod” is to place a concrete paver block on top of some isolation pad, typically made of rubber or sorbothane to increase the functional mass of the printer and lower the resonant frequencies created during printing.
I personally have two of those enclosures stacked, with a printer in each one.
I just bought a concrete paver and neoprene mat to put on top of my lack and I'm printing pieces for my lack enclosure on my ender 3 as I type this message.
One thing I'm not sure about is whether or not I should attach the printer directly to the concrete paver, and/or if I should take the rubber feet off of the printer as well.
If so how do I best attach it?
I'd just try the easy thing first and set the printer on the paver without attaching. See if that works before you go through the effort of attaching it
They are drop in replacements for the existing board and are very affordable. I highly recommend them.
The least-stable arrangement would be a very heavy 1u device at the top.
Here is a guide by Prusa to build one for their printers: https://blog.prusa3d.com/mmu2s-printer-enclosure_30215/
I know of a few places that just procured literal blank marble gravestones as a "base" for their analytical scales at a cost far less than the usual mass-damper bases sold to labs.