I use a donut bobbin holder for my L-size bobbins for my Janome sewing machine.
But I also have M-size bobbins for my longarm machine. They fit in the plastic-tube holder, but the threads constantly unwind from the bobbin.
I looked for a better M-size holder while at the Quilt Festival in Houston, but didn't find anything I liked. What I did get, though, was a great suggestion to use ponytail holders. No more unwinding threads!