In days gone by there was a red masking film for graphic arts use called Rubylith that might be suitable, but I don't know if it is still made. Another possibility might be sheet acrylic from a sign maker if it can be had in red. Not sure how reliably safe it might be. Also sunlight is pretty bright, you might need to further attenuate the light. I would tend to opt for an opaque shade and separate safelight.