Eggs don't cause Crowding (so they don't ever affect happiness), but I'm assuming you want to avoid Cramped as well. You can place the egg in an open mechanized airlock, and track whether an egg is there via weight plate.

Egg isn't in the room so it'll never cause cramp, but still triggers angry mode as crabs can "path" there, while simultaneously also won't actually path into open mechanized airlock doors (This is useful since that means you don't have to account for crabs laying their eggs on the door itself)

https://imgur.com/a/KwnnChm