Fortunately there is just over 10mm of space in front of the LEDs on the hotend, more than enough space to slide in something to add a bit of colour.

Printed with settings leaning towards this, though not as critical to get it all the way to glass-like.

You might have to change the thickness of the plate depending on what transparent filament you use, but just keep the tabs at 11.18mm to hold it in place.

Update: 04/23 Added a 1mm thick plate that works with solid color PETG, thanks to AH - Prusa i3 MK3S+ Kit@AHPrusai3MK3SKit trying it!

(For the purple I had to take the picture up close and out of focus because the camera didn't want to show this electric arc kind of ultraviloet colour of this Zi-Rui PETG Filament that they describe on Amazon as ‘Sparkly Glitter Purple, Shining Glow in The Dark Green’)