existing mass produced bags that might serve as design inspiration, or adapted/adopted as is with minor changes to fit your particular bicycle:

https://www.ebay.com/itm/182161551435

https://www.ebay.com/itm/401966404798

https://www.ebay.com/itm/401965144283

https://www.ebay.com/itm/402436360069

https://www.ebay.com/itm/401770792026

https://www.ebay.com/itm/224141743779