What is recommended for a ported sub. my box has an internal volume of 5.3 cubic feet after taking out driver volume, braces, etc.

titanic 15
500w bash amp
2x4" ports 28inch long

What is recommended for a ported sub

None up to some amount that degrades performance, lol

To understand the topic better, try it with no stuffing for a while to understand
the sound you hear, then add some loose stuffing and do another comparison.
If you add too much, ie filled all the way up tight, you will degrade performance alot.

I call this user perference based on a few experiments.

Start by lining the interior walls of the box with a single layer of R-13 fiberglass insulation. Keep the damping well away from the ends of the ports.