A few times recently in Sokoban, I've been having issues with inconvenient monster spawns that end up blocking my boulders. I most frequently run into this trouble when I'm nearing the completion of a level, when the level's long "pit corridor" is almost entirely filled in. Although I've verified the hallway as being fully clear of obstacles ahead of time, by the time I walk back to my boulder and push it all the way through the passageway, occasionally a monster has suddenly appeared in the hallway in front of the remaining pit(s) and I get stuck.
The only thing that I can figure is happening is that a monster is spawning in the passage after I check it, but before I'm able to get the boulder to the pit (with no real opportunities to re-check the corridor as I move the boulder due to the structure of the level). I can always destroy the boulder and use/create another, but I'd obviously like to avoid the Luck penalties if possible.
Is there any good way to mitigate the odds of this happening, and/or fixing the problem when it does? Methods that would be reasonably accessible to an early-game character (and, ideally, without assessing Luck penalties) would be preferable.