Here is an even more ominous answer for you: It all depends on what helmet and helmet size you are using.
I have an overall very small diameter shell racing version helmet. It's a size L Arai, but because it's a racing version it's really quite a bit smaller then other L Arais. It is much easier to get it into small top boxes or side cases then other helmets I've used. The other extreme are some of the bigger flip face helmets. Is that what you mean with Modular? They are sometimes so big that you'll have a problem with most or all of the smaller boxes. My point is you might need to take your helmet and either go to a local MC gathering and ask somebody that has a box that looks interesting to you to let them try your helmet, or go to a MC store/dealer that has some on display.