Best avoid large light sources like light boxes when copying! Your best bet is small strobe reflectors with polarizers as far from the art as possible. Also try a shallower angle, instead of 45 degree. And of course polarizer on the lens. Test to make sure it is cross-polarzing !
I'm working in a very confined area, about 10' by 10'. The shop I work for had to move into a smaller building a few years ago and space is quite tight. (They used to have a larger room for this purpose. Now they have a 10x10' corner of a larger room. At least the studio area is painted black and there's a bit of a partition.)
When I was hired they seemed to be using narrow tall soft boxes all the time, but they have umbrellas and small parabolic reflectors. I figured the sort boxes must be better for avoiding falloff and hotspots at short distances. Are parabolics going to work at distances of 6'? And if I need an angle of less than 45 degrees then the max distance I can move the lights away is going to be less than 5 feet.
So I have two major restrictions: space and this isn't a very leisurely work environment so I don't have much time to experiment with stuff. To give you some idea, this is a general purpose imaging and digital printing shop that tries to do it all, so the photographic copy work is just one small part of the main business.
I'll have to search around to see if they have any polarizing gel. (The previous guy quit before I got there and everyone else there seems rather unfamiliar with what photo accessories they have and where.)