Try to locate an art supply store that sells small individual sheets of Rubylith masking film. Cut out small pieces and stack as many as you can over the opening while still being able to read the frame numbers.
Note also that on my Brownies I still cover the red window with a piece of black opaque photographer's tape between frame winds, just to be safe.
This stuff is very inexpensive. If you can't easily locate any, send me a private message (PM) with a safe postal address and I'll send you a small sheet.
[Edit: Oh, and welcome to APUG...]