I have the 3.5 gal Rise Kit Pro which is battery operated and I can say that it is awesome. I use it to rise off my surfboard and shower after surfing. I also have the optional heater element which warms the water really well. If you are on the fence about it I wouldn't hesitate to buy one. It is a little pricey though but worth it in my opinion.
If there was not such a price difference, I’d go with the longer one but wouldn’t put it on my rack. I’d put it on top of the Decked systems up against the cab end of the bed, running crossways perpendicular to the bed. Super easy to get to all of the controls and the outlets and the weight is still kept low.
edit: I just looked at their site and they don’t have (at least not yet) a rack mounted version. Their 4.5 gallon looks like a long Yeti soft cooler.