The ones chemglass makes are the bees knees, although it is probably more expensive than the traditional coupler.
https://www.neobits.com/chemglass_cg_2044_50_tru_stir_stirrer_shaft_p4141857.html?atc=gbp&gclid=CjwKCAiA4KaRBhBdEiwAZi1zznsCUbw-M2E1HHYNOaNLkr1hxY3PVgSkvwwpTMBJALqWo9TXtbLuVRoCh9AQAvD_BwE