This design is made for my bike but should fit most bikes.
I used a tool like this to get the shape of the down tube.
There is also a Fusion360 file and a .stp file so you can edit it to fit your seat tube and down tube perfectly.
 

The box has quite some potential. “Unfortunatly” I am so happy with it that I can't find motivation to make it better for others.

Some Ideas:

• add a drain hole 
• make it ajustable for different frames
• make it prettier
• add a mount for connection to the down tube
• change from zip ties to printed mounting system
• make it removeable
• ….