Several people here have made reproduction inserts / brights for Roberts, Dynatron etc. knobs -
https://www.vintage-radio.net/forum/...ad.php?t=26740
gives plenty of detail on how to go about it - and they've also been offered on eBay etc.. They're concave on these knobs, but I think they're the same as those presented convex on older models, just fitted upside down.
Paul