Suggest a top of the rear rack, clamshell opening, semi-rigid bag?

Okay, people of Reddit - what suggestions do you have?

Looking for a clamshell opening, lightweight gear box or bag that is a duffel, tote, or backpack style that I can strap to the rear rack.

Yes's:
- 20ish L capacity
- Must have a clamshell opening
- Semi-rigid, so it doesn't flop around too much, and things stay put
- Would love a laptop sleeve, but not 100% necessary
- Can carry around off the bike

No's:
- I don't want a dedicated trunk bag
- I don't want a messenger or commuter/briefcase type bag
- I don't want a basket (just want to strap it down)
- Have panniers, love them, just want something different for the top of the rear rack

Patagonia black hole mini MLC, Fjallraven high coast duffel 22 seem to fit, but they aren't very rigid. But that's the type of dimensions I'm looking for. Help?