I tried a flat board, no go. A cold light may be the best solution. I used one for years on my beselar 23cc and really liked it. Magic it was not, but I did like the prints I made with it.
I have an inquiry into aristo right now to see if I can use the same set up, but change the lamp to one that will cover 4X5. I may end up just adapting the cold light I've got, use the condesers for 4X5.
I also noticed in my playing, that if I raise the condenser off the carrier I can get even coverage. Maybe I need to take a light meter and measure how high I need to be to get even coverage, and then make a collar that length.