Step 3: Hollow Man...
Using a wide-blade knife, chunk of metal with a flat bottom.... or something spatula-like, start taking chunks of the foam out of the bottom of the cooler. Be careful how much gusto you put into this- eventually you'll hit the inside liner of the cooler. If you break the inside liner you're gonna have a bad time.
To boost space, carve as far into the space between the inner liner and outside shell as you can, around corners or whatever.
Chances are you're going to have a cavity that is 1" deep. That's not going to look like a lot, but 1"X12"X12" = 144 cubic inches = 2.3 liters, more than a handle of your favorite brown liquor, or a growler of beer, or 3 bottles of wine. Math is badass.
At this point you can test how much room you have by filling a ziploc with water. Using an empy wine bottle (0.75L, the same size as a regular bottle of liquor from the store) see how many bottles you can fit in there and still fit the lid on easily.
The ziplocs will let your beverage conform to the room available. Science!