I modeled 5 cu.ft. tuned to 18 hz. Since you want to do a sonosub, you'll have to be creative with the porting. To keep the airspeed under 26 m/s you'll need a 5 inch diameter port. A 5 inch diameter port will give you enough room between the end of the port and the back of the sub, if you use a 16 inch diameter tube. With a 4 inch port the air speed is to high, with a 6 inch port the port length is to long for a sonosub. I'm not sure if sonotubes come in 5 inch diameter. If not you could build a square port with the same surface area.
The sub will handle 650 watts with a subsonic filter at 15 hz. Here's the SPL graph of the modeling.
Here's a screen shot of the Sonosub software showing the dimensions of the modeled sub. The TC 2000 sub was in the database in the software, I'm assuming your subs mounting depth would be close to what is shown. The internal diameter of the tube is shown as 15.5 inches which is what a 16 inch tube would be.