That extra bass strap is now in a drawer and I'm mostly keeping my left hand in position. I feel like I've graduated from beginner school, in terms of needing a training aid, at least. I found that using the extra strap and applying advice from other players helped me sort out why I was drifting all over the bass plate, so thanks for the replies.
In summary here's what I learned:
- My bass strap was way too loose. This in turn created several effects as I tried to compensate.
- My wrist watch interfered with positioning my wrist under the bass strap. As a result my hand was too far back and the bass strap was against the back of my hand. Tensioning the bass strap pressed against tendons and caused discomfort. When I did put the bass strap over my watch, the watch strap reduced the friction between my wrist and the wood, allowing my hand to skate around.
- To create more friction I was twisting my wrist to press the base of my thumb against the box. That pressure created muscle cramps. (Others prefer using the base of the thumb in this way, so this method probably would have worked if over time I had learned how to optimize the pressure.)
- The box was leaning back too far from vertical toward the axis of my torso. With each push-pull against a strap that was too loose, gravity tended to pull the strap downhill toward my elbow. Keeping the back of the keyboard closer to vertical or tilting it forward helped resist this.
- I was not taking the opportunity to push gently down when the bellows were nearly closed, using my leg to resist the downward pressure and thereby resisting the tendency of my hand to drift upward toward the air button.
Now my watch lives on my right, the bass strap bears on my wrist, the box leans a bit forward and I push my hand down when I can. Gradually all this is becoming less conscious and at times it feels just like having the extra strap but without the bother. Whew.