I am working on a design to mount a 4x4 plastic tube there. I will make it waterproof (edit) water resistant with locking latches.
I will share the design once I get it all together and tested.
Ok, have attached pictures of what I have done, not that I am finished yet, but concept is working well.
On the under basket storage unit, I have glued shut one side, the other I will be installing two latches on two sides to latch and hold the cover on. These latches have eye loops so I can put a lock on, should I want to.
The front square tube has a small hole drilled in the bottom for drainage. I have a lid/cover for it as well, but no latches are planned for this one. I plan to use it for drinks or whatever.
Now, on my wife's bike, she wants me to paint her storage tubes black.... not sure if I will or not.... I prefer function over what it looks like.