My accordion is quite a heavy instrument, and the outer casing is smooth. I usually practise seated, and like that, with the weight on my left leg, I have no problems playing. When I play standing up, the left-hand side of the instrument tends to slide down through my grip when I'm playing, making it harder for me to finger the bass buttons. It's not too bad when I'm pushing the bellows in and the weight is on my hand, but it's a big problem when I'm pulling the bellows out, which then makes me switch direction too soon, messing up my phrasing.
I've often heard recommendations to wear something like a sock with a hole cut in it as a sleeve, as an aid to moving one's left hand around. I've tried this but it makes the instrument more slidey, not less.
How do I make the bellows stay in my hand so I can support their weight but still move up and down the button board when I need to?