[Aavso-photometry] Light box question

[Robert J. Modic]

I've been using a light box to take my flats with for a few years 
now and am generally satisfied with this method.  Twilight flats 
are problematic from my site due to weather and time constraints, so 
the light box gives me more flexibility.  However, there are sometimes
residual artifacts in my images made with light box flats, especially
in stacked images.  These are usually at the ~1% level and affect 
mainly the edges of the frame.  I'd like to improve on this.  

Since my L.B. uses tungsten light bulbs which emit a lot of IR, there 
is a mismatch between that and my night sky which has a fair amount of 
light pollution and, on some nights, moonlight or twilight.

I'm looking for some material that I can place over the front of the
L.B. to make its output more neutral (white).  Perhaps a light blue
plastic sheet might accomplish this.  Does anyone have any suggestions
for specific materials that I could use for this purpose?

Bob Modic (MRV)