Well, another question would be, are those dimensions max dimensions you can have? your box volume may need to increase depending on the port size you want
The 13" measurement is slightly flexible, but I want to try and keep the other two measurements the same due to the shape of my hatchback. So how long should my slot port be? I'm not sure if you are suggesting the 2" round ports or if you are saying my slot ports should be 2" in width.