I seen a couple guys use a rear cooler from an edge and cut off the side tabs. Then it fits right between the existing coolers. You can mount it underneath and you wont even know its there, just add another piece of hyfax on the existing tunnel protectors and your good to go. This is what im planning on doing soon.
I put in the bulkhead cooler like a regular rmk has (you will also need hoses, hose clamps, rivets, and coolant bottle). I did the whole project for under $150 with used parts and it made a HUGE difference. You will need to pull the skid and drivers out, but the whole project took about 2 hrs.
Get the ftx one. You just cut the cross tube thats under the tunnel under the tail light and put elbows on that direct the coolent through the extra cooler couldnt be easier.