Google "hydro flask flip lid review" and watch the negative reviews pile up. This is a shame. The hydro flask is a great insulated container, but the lid is a joke. The cutout in the back of the lid allows coffee to spill down the back when you actually try to use it as a lid to drink from.
Luckily, this is pretty easy to fix.
Step 1: Sugru
Sugru is a mold-able sticky putty. I picked up a pack of three (black, red, and white) at Ace Hardware. You find it online and even at Target.
Open the packet and knead the putty a few times.
Use the putty to fill the missing lip from the cap.
Step 2: Let Sugru Cure, Then Drink.
Once you have putty in place, let it sit for a day to cure. The Sugru hardens into pretty tough, water tight plug. I have been using my repaired lid for a few weeks now with no problems.