KEMIMOTO Motorcycle Tail Bag 12.5L Waterproof Expandable Saddle Bag - Multifunctional Storage for Motorbike Rear Seat, Tank & Outdoor Sports Motorbike Accessories, Black with Detachable Shoulder Strap

It’s a Kemimoto tail bag. It holds that shape and expands at the bottom if needed. A little pricier than most you’ll find but worth it if you don’t like crushed hats. 😆 KEMIMOTO Motorcycle Tail Bag... https://www.amazon.com/dp/B0DXL1XWRP?ref=ppx_pop_mob_ap_share

I have something kinda similar. Installation is simple and it works pretty good. KEMIMOTO Motorcycle Tail Bag