I use a 2x2 cardboard cup holder in the bottom of my left side bag. The cup snaps firmly into place in one of the drink pockets, and the cardboard holder (FREE with purchase!!) is only slightly compressed by the lid into the back of the case.
Panera Bread Co. is about 1.5 miles from work. I usually sip of about 1" from the full cup while enjoying a breakfast bagel. No spillage inside the case - and no wind siphoning problems either.