I just did one of these not too long ago here. Mine was the marine version, but shouldn't differ as far as the fittings go. I used the 1.5" cooler kit from Bargain Fittings and it worked fine, no leaks and turned out reasonably sturdy. I reused the stock gasket, and added the kit's washer then a coupler on the inside. It's a tight fit, but leak free. No modifications were required to the cooler. 2" would probably work, but might be a wee bit too long, requiring additional washers for shims.
I do think I might change the vessel's fittings up a little bit, but mainly because I feel my valve's handle gets too close to the cooler in the stock configuration. I haven't decided how I'm going to do this, whether it'll be with a 2" nipple and shims or add a couple/nipple on the outside. Good luck!