How do I design the top part?

Hello, I bought replacement bins for a pen
(Amazon Link: https://www.amazon.de/gp/product/B0793M87N7/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1),
and I would like to design myself a new type of cover for my pen.
Because I am relatively new at CAD. I don't know how to design to top part (Picture: f1.png)
Any suggestions?
Thanks