One of my shooting associates filed the rear sight blade of his "rifle-sighted" Remington 870 barrel into a shallow-V "express" configuration several years ago. He shoots it well.
I prefer ghost-ring sights on a "social" shotgun, but must admit that they are not much better than a good set of "express" style open sights.
A couple other friends of mine have fitted their shotguns with a square front post and square-notch open rear sight. Their resulting sight picture mimics the sight picture they are accustomed to on their pistols.