Need help with port length / size based on my enclosure

I will soon be embarking on my first sub build, and plan to put the enclosure inside a current wall void. The internal box dimensions that I came up with are as follows; 1.16' wide x 3.256' tall x 2.08' deep. This nets 7.9 cu ft for a Dayton rs5390hf. Don't worry about room between studs. I will handle that.

Since this will be "in wall" the driver and port will need to be on the front face. What size and length port do I need to hit 18 Hz?

Thanks!