General description:

If you put your LACK enclosure tabletop, there is no place for my favourite spare steel sheet holder:
https://www.prusaprinters.org/pl/prints/51462-expandable-steel-sheet-holder-mk-mini

So I've designed a set: smaller (square) for rear and a bit wider for front (just to made holder installation easier). There are two versions for front feet - v2 is a bit narrower (safe for large nozzles ;)

If your table isn't perfectly level, adjustable feet are welcome - there is a room for feet with M8 screw (i. e. IKEA LÄTTHET, btw: you'll find TPU pads for them here: https://www.prusaprinters.org/pl/prints/77128-ikea-latthet-tpu-pads) - you will need 4 M8 nuts.

Printing:

Filament: any, PETG preferred. 
I've printed with following settings: 0.6 nozzle preferred, 0.3mm layer height, 3 perimeters, 15% rectilinear infill, no supports required (but putting nuts after print is done may require some heating due to a space limited by bridging - a pause for nuts recommended, but not critical).
Only front left feet is included - make right one using mirror option in Slicer. Rear feet should be printed twice. Whole set fits on Prusa MK3, but if you fail,