Hey Yakfreak,
Here is one of my posts from 4 years ago detailing all the cooler options that will either fit in, or ride on top of, the tankwells of the earlier OBs. Don't forget that you can always use a "spacer" plastic box or flat tackle box to put in the tankwell, and this opens up a number of additional coolers that can then ride nicely on top of the "spacer" box. One that I like for this is the Coleman 28 quart Ice Basket Chest Cooler. But you can also use the Igloo 25 as well, or any other that will fit between the gunwales.
http://www.hobiecat.com/community/viewt ... ice+chests