I don't know about the Igloo coolers, specifically, but the 10 gallon orange Home Depot and red Rubbermaid coolers are smaller than the 5 gallon orange Home Depot coolers. The red Rubbermaid 5 gallon coolers in my local Home Depot are also the smaller size. The smaller size needs 3/8" fittings, while the 5 gallon orange cooler needs 1/2" fittings.
For the 5 gallon orange cooler, I used the stainless fitting set from Bargainfittings.com, after I couldn't get a good seal with 3/8" hardware. This will fit a 7/8" to 1" hole in the cooler.