You can use PMMA fiber optic threads. Often times you’ll see them advertised for “Star Sky Ceilings” and such. It’s not terribly expensive, on Amazon 50M is less than $20 in the US. You’ll find it even cheaper on AliExpress.
You will also want to make sure that you get “end glow” fibers and not “side glow”, as the side glow will leak light from other LEDs around it and be dimmer. It’s also possible to order them with a PE jacket which could help with any light bleed issues in the tight confines inside the shell, but it could make routing the fibers harder as they can’t turn as tightly.
Another option is “light pipes” (also called lumirod). They’re similar to the fiber optic strands but are not flexible at all. For the front mount LEDs that are a straight shot, these might be easier to use, but they’re a little harder to find, as they’re mostly used as optics for firearms and colored in green, yellow, and orange.
If you used a light pipe you could use LOCA to glue the light pipe to the front half of the shell, which would make disassembly easier vs fiber optic threads which aren’t as rigid and would require being glued to the front and securely attached to the LED light source as well.