Rubber band or o-ring rolled up from the handle onto the hair to make it act like a shorter loft, take it off to dry.
I hope you get it sorted. It's a nice looking brush.I have a brush that is far too floppy. It is ~19mm but set at 60mm.
Not only do I like the handle, but I love how soft and comfortable this vintage brush is and would love to work out a solution.