For sure, pannier bag. I use Topeak MTX Quick Trac. Not as capacious as what you describe but super handy because it's a bike bag with drop down panniers. I've been using these for years and the only beef I have is with the zipper. It doesn't last as long as it should. I'll def keep this in my reading list for next time I want to change my kit. Thanks!
I see you are being dinged because this is so much like an ad. Oh well you can't please everyone.