Strange request , As I get older , I find that due to failing eyesight , I need wider slots at the rear ...
Best advice I can give is that you try to cut a very good copy out of an old coke can , paint it matt black & fit it onto the original sight ( double plate, the screw should not need to be replaces, as these cans are microscopically thin these days.
If it works, then leave it , if not , you have not bodged or otherwise damaged the rifle .
When we shot Military Bisley many years ago , with open military peep sights , we used to dab a blotch of nail varnish into the hole. This would stick around the edges, making the peep hole minutely smaller, a liberal coating of sight black afterwards , & your sight picture was improved , without illegal work to the sights.
The rules said you could paint them if you wished ..... Well we painted them , then blacked it ....