I like rack trunks. But if you're commuting, you have a lot of things to carry, like lunch, change of clothes, rain gear etc.

I couldn't find a trunk that's 20l in volume and backpacks or duffels strapped to the rack are too cumbersome.

I now use an Arkel pannier/backpack. The advantage is that when I over stuff it and hang it off the left side, it looks bulky so motorists tend to give me more clearance. At least that's what I like to think.