After my bike trunk bag when flying down the road last summer I picked up a Topeak beam rack and MTX trunk bag. The bag has a rigid base that slides on and locks into rails on the rack, making it sturdy, yet easy to remove. No more velcro.
The Topeak trunk bag is tough, waterproof, and has generous fold-down panniers which are there when you need them, tucked away when you don’t. It has quality construction with Teflon water protection and rigid foam sides that double as an insulator. Push the disconnect button on the rail and take it with you,using the built-in carrying handle, plus detachable shoulder strap.