Well, if you insist on installing a dovetail rear sight, it looks like the Novak Lo-Mount for the Colt .380 might fit your needs. The sight width is narrow enough to fit on the top strap while the dovetail appears to be deep enough to eliminate the pivot pin hole, while not so deep as to hamper frame strength.
As for filling in the frame slot, fit a piece of metal into the slot and hold it in place with a screw into the hole for the original elevation screw. Depending on the placement of the Novak sight, the rear blade might be long enough to obscure the screw head from view.
As for filling the slot with epoxy, YUCK! If you want a nice custom handgun, don't do a half-assed job.
By the way, Hamilton Bowen has a single-action revolver on his portrait page in which the rear sight has been repositioned. I suspect some serious rewelding and reshaping took place.