Hey Nasher - you've been quiet lately!
Sorry I'm not a RIB builder!* Are you concerned about what the void may do or what may happen within the void? My concerns would be "can it leak and fill with water?" and "then what?". You could pack the void with expansion foam or a polystyrene block. I'd favour the block as it won't break down or absorb (much) water, while maintaining the buoyancy of the void.
I have the same problem in my rib and live with it. I tell myself that extracting the two anchors and four lines etc. for drying, keeps me familiar with the contents of the locker!
It doesn't really help...
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* not that sorry, actually.