Sometime paint work is an option on a thin veneer to mimic the tortoise shell. Or maybe a resin veneer?

Try searching for "tortoiseshell pickguard". Something like this will pop up in a range of colors.

https://www.etsy.com/listing/1475747636/tortoiseshell-amber-acrylic-sheet-25mm

I wonder if you couldn't photograph the existing shell and have something fabricated from that photo? I don't know what I'm talking about, lol, but I'm thinking like even a thick paper or something a 3d printer could print...or?? Like they do with fake wood floors where they finish the top with photos of real wood?

Or you can try searching for 'tortoise shell celluloid sheet'/ 'tortoise shell acrylic sheet'

I like to use Google Image search, and go from there.

https://imgur.com/a/ol30s2I

I'm not having much luck finding any faux shell that's even close to this red. I think I'll have to resort to some type of filler that I paint to match the red/black but I don't know what type of filler will work with this. I believe the wood/brass/shell combo does move quite a bit based on temperature and humidity so I'm concerned about the filler cracking.

Paint the back of a clear sheet with red and black. Cut to fit.

It's practically impossible to match a color from photo images on the internet - way too many variables at play. Are you only seeing these virtually?

Is there any way you could order samples so you can see them in real life? Hubby gets samples of plastic laminate for drums that way.