Sheet effects will help you here. As long as your pit floor is on a different sheet to your main floor you can use a glow set on one of those sheets that is set to either inner or outer as appropriate to the sheet so that the lower level has a dark edge that fades into the centre of that floor.
Comments
Dungeon map symbols under trap should have a pit cover or pit open symbol if you want to go that route.
Was on my cell phone last night. Took out one typo today.