How much 'bowing' do you have? Lately I have been fooling with glass negative carriers, particularly for 35mm and closely inspecting prints I have made over the years with glassless carriers (Beseler 45MX), and I am unable to detect unsharpness in the print made with the glassless carriers. I can see that the negatives are not held flat, and I know glass will keep them flat. But given I haven't been able to detect a difference in the prints I wonder if the glass carrier is worth the bother for 35, at least. For 4X5 I have negaflat, and 2 1/4 the Beseler Negatrans with glass does quite a good job. But this problem is the same as yours; how accurate do you really need to be? If you you can't see a difference in the print, why worry?