FWIW, I don't "lose" apertures because I remove them before I go hunting, and use the resulting "ghost ring".
As far as barrel-contour slot fillers go, some like them, other don't - but nobody sells them commercially, they're usually self-made:
Those that don't like them often file a fancier one out of the dovetail section of an old rear bbl sight:
YMMV.
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