I think you will run into issues with that letter box board… you can see the unevenness in the light even just in this picture, which will lead to discolorations in your negative inversions. Also, since CRI is not a consideration for letter box board, it will not be able to reproduce color accurately.
More information on what to look for in lightbox here: Suggested backlight sources for scanning film with DSLR
-Nate