I'm looking for something to fill the space/hole that is left when you take the rear sight off a smith and wesson 686. I don't want a rear sight. I just want it to flow smooth and flush with the rest of the frame. I've tried looking for an insert online, didn't have any luck though. Maybe you can help.

Welcome to the Forum JxPullie!
I do not think there is a product on the Market to fill that void but any good Machinist could make what you desire.
As a suggestion, a model 65 or 681 will give you the caliber and the top strap you desire.