Offered with due respect and just want to help....
I don't think there is any use to pursue those ideas and it sounds like a butcher job (but not venison) in the making.
You can probably glean just about as much accuracy by using the existing bead to sight with. (I don't like doing that, but it can be done with fair results.) O/U's are not meant to be used that way and depending on the gun. choke, the barrel convergence, the slugs you use, etc. your useable accuracy is pretty iffy. Good single barrel slug guns are very inexpensive, will come with proper sights or scope mounts, and some have stellar accuracy. Lots of used ones available too. Maybe buy a used NEF shotgun and send it in for a slug barrel with sights - they are amazingly cheap and accurate. Put the money you were gonna spend for a rear sight and hot glue towards one. Silicon or hot glue will not hold a sight on.
Lots of inexpensive muzzleloaders with good sights and accuracy potential too nowadays too, deadly deer medicine. Good luck with whatever you do.
