Were you able to find a narrow cargo box? Are you putting the box on the factory cross bars? If you are still looking to find a box but can't find one that narrow (looks like Thule and Yakima's narrowest ones are around 24") you could remove the cross bars and purchase a set of Thule 450 crossroads feet and a pair of Thule 65 inch square crossbars (or whatever the Yakima equivalent is) and you should get enough bar length to accommodate both a bike carrier as well as many boxes. It may look goofy and may be a hazard, but legally you can have the cross bars extend out no further than your side mirrors, so you can have the bars even longer than 65".
The disadvantage of going with the square bars is the impressive annoying howling sound it creates when the cargo box is not installed. I couldn't figure out an easy way to install a fairing with our vintage Thule Cascade XT box on our van--the box can't rest on the front cross bar if the fairing was in place. Since we don't need the cross bars and feet most of the time, they are stored inside the cargo box in the garage and not kept on the Odyssey.
Hope this helps or that you found a different solution.
Malone makes very affordable aero cross bars that will attach to the factory rails and give you plenty of room for things on the roof. I have the 65" width ones on our Odyssey. Our roof box is narrow (old-ish Yakima Rocketbox I believe), and leaves plenty of room for bikes on the other side (probably would fit 3 up there pretty easily).