Merry Christmas all!
The other day it occurred to me to try out wearing a wristband on my left wrist to keep the arm in contact with the case of my B/C/C#, and, indeed, it gave me a bit of extra power/control. This might come in handy for those who play big boxes, or smaller ones with lots of voices.
For a wristband I actually used just a sock with its toe cut out. I tried a friend's B/C which he uses a long strap with and it's an easy box to play in the first place and my wrist was in full contact anyway so this might not be a big deal for those who play with the box out on the knee. Mine are almost all held close to my chest, and I can contort my arm a bit to keep it in touch with the case but a wristband does this for me. I've practiced for hours on end without any discomfort.
I've read how many here wear wristbands to keep themselves from carving their skin up on Pokerwork cases, too.