I responded to your post in the general flashlight discussion forum, but check out Butler Creek Blizzard scope covers and find some quality red film to make your own.
home depot, lowes, ace..... pvc caps and fittings, pvc pipes, when you heat them up, you can shape pvc very easily. Other than that without 3d printer plastic container bottle caps, you may find one that will fit,
as far as red glass, look for polycarbonate/acrylic sheets they come in many colors.
I responded to your post in the general flashlight discussion forum, but check out Butler Creek Blizzard scope covers and find some quality red film to make your own.