They use router bits for slots and those are, obviously, round. So you can have square outer corners but rounded inner corners but that’s ok as long as the flat part of the slot has the same length as the board you put through. As for the wiggle room, the board thickness may vary a bit, mainly with the finish. Immersion gold is more consistent, hasl is bumpy, for example. For my projects I make it tight and, if needed, a small file does wonders.