Preferably I use nothing on my head. In the video I use noseclip because duckdiving floods my nose and prevents me equalising. However I did dive a free immersion dive in Spain, only wearing speedo's and nothing on my head, feet first and it was a delicious near flow dive.
Now for static preparation I wear nothing. I simply lay on my back floating, breathing very slow and relaxing. Having the energy visualisation paired up with - like you do - very slow nasal breathing works wonders. When I'm rested comfortable, warm relaxed, I can feel the mystical flow feeling hitting me within a few shallow slow breaths. It used to take a while, but I guess training helps
When I do this kind of preparation / meditation, I feel my limbs "light up", as if they loose weight and start to radiate light from the inside. This sensation starts with the legs and goes gradually up to the arms etc. When my whole body is lit up, I'm ready for a 'killer' static
LOL It usually takes about 12-20 min to reach that level. At the end I'm ventilating amazingly small amounts of air.
Even sitting here behind a desk, if I inhale slow and shallow wile applying the lease amount of effort I can find my legs already start reacting after just a few minutes.
The pauses at top and bottom of the breath are for me very rounded ones, more like a sinus curve matching airflow. Even though it's easy to have a flat stop I prefer a fluid small movement, perhaps breathing shallower and or slower.
Play around, and I'm looking forward to your experiences and sensations.
BTW flow appears just to happen, but when I forcefully look for it, it often stays away or is less deep. Maybe similar to falling in love, therefore I like the falling in flow phrase