Front Rack / Porteur Bag

Hello,

I hopefully looking an used front rack bag.

Perfect bag would be:

• 20-40 litres capacity
  
• Stiff enough to hold the shape when empty
  
• Waterproof
  
  

So I imagine something like Swift Industries Polaris or Ozette or Chrome Industries front rack duffle would be perfect as a bag. Or something custom like this: https://www.lfgss.com/conversations/274702/

Thank you,