You could probably get a Fiamma box or bag to sit up there. We have a small rack up there and stuff can be stored up there with a net and bungees but we just got a box from b&q and strapped it to the bike carrier.
David.
They come in 3 sizes are cheap and have a lockable lid. I cut some slots in the bottom and in the back so I could use some straps to attach it securely to the carrier. Any gaps around the slots/ straps were filled with araldite to make it 100% waterproof again.
David.
Like the idea of alternative use for the bike rack, I sometimes have used mine to carry the awning and poles in their bags especially after a soggy week away, bit wary of the extra weight in contrast to a couple of bikes though.