These are standard thread sizes and manufacturers often use standard size screws to attach bases to a rifle. To make a plug get a properly threaded bolt or buy a die and make your own. Cut a section that will protrude a slight amount on both sides. Lightly chamfer the holes, both sides?? and screw the plug into place,flush. With a light hammer and flat faced punch peen the ends over to fill the chamfer. This will become a permanent filler and if filed flush and touched up will be quite unobtrusive. You will still be able to see it but it won't be as bad.. If making a removeable plug cut the pieces of bolt a bit shorter than the holes and screw them into place in to a correctly threaded hole in a block of steel or the receiver itself. If you use the receiver be very careful. Peen the head(out side end) a bit. Cut a slot with a screw slot file or hacksaw blade with the kerf filed out. Screw the plug into place and completely fill the Chamfer by peening the head. I use a polish faced flat punch and a very small machinest hammer. After the chamfer is filled remove and file the head into shape, polish and blue with a cold blue solution. Be sure there is nothing protruding into the receiver that will interfere with moving parts..
