You might want to use a black plastic bag (like ilford uses to package their film and paper) inside the three tiered box as additional insurance. These are available, for example, here: among other places.

I have never seen 3 tiered boxes for sale, but they are the detritus of the large format shooter. If you know anyone else who shoots 4x5, they will have several boxes lying around. Make sure you mark which one is "Exposed C-41 only" on the outside. (or E-6 or what have you).

One thing you can do is buy two boxes of film and put the unexposed film into one box. This works well with 10 sheet boxes, which are the same size as 25 sheet boxes. Kodak 50 sheet boxes are the same size as Ilford's 100 sheet boxes, also.