This is a 6th order BP type design. You don't want stuffing, just line the walls with some 1.5 or 2" / 50mm thick foam and staple gun it in there. Do the ported section only. All you are trying to do is knock down some of the high frequency reflections outside of the sub bandwidth. Try sticking your head inside of the unlined cab and snapping your fingers...There will be a lot of HF's and reflection. Repeat after lining and it's much much deader. Also it keeps the wire from rattling if they end up touching the cabinet wall. Keep the port entry clear though. A good 6" / 150mm clearance or so should be fine. See pics below.
For a particularly nasty resonance you could put a pillow in there somewhere but it'll cut some efficiency and you must keep it away from the motor vents and the ports both.
For calculating port tune I never include any extra length past the front plane of the port exit.
When the exit is close to internal walls it will result in an apparent lengthening / deeper tune. This is where the extra apparent length comes from. See the line through the middle of the port section in the sketch below. Sometimes I use a radius in the corner in place of the straight angle, but this way is slightly more conservative. This is how I calculate them in cases like this. No extra length beyond the external exit. There is an extra few cm beyond the internal exit due to the back panel. A 45deg angle seems to approximate this length well based on measurements.