At work, each tech has his own 500ml brake fluid bottle, usually quite new coz we get through a lot with constantly refreshing brake systems.
So it's common see an old trick here in NZ workshops, the plastic lid of the bottle has a hole drilled in it and a tubeless tire valve pushed into that hole, sealed around the lip of the tire valve with the correct sized hole drilled in the lid.
Remove the valve core from the valve and add a 6 inch length of rubber or plastic hose to the end of the valve, fuel hose works well, and that's a great permanent addition to the workshop toolbox!
Then when you open a new bottle of brake fluid, fit the hosed lid. Easy!
Makes it so easy to pour into rear brake reservoirs instead of using a syringe with a hose like I always used to.