Shimano

SM-DISC-BP Oil Funnel Tool with Stopper

Works well with Simano brakes. Screw this into the reservoir cover and then push fluid through the caliper bleed port using a syringe and tubing (not included). Takes maybe 10 mins, works the first time, does a pretty good job of minimizing mess, and it can be done without a second person although it'd be easier with help. I've tried bleeding brakes tops down without this thing and it's hit or miss. Definitely worth the price.

Just buy it and bleed your brakes, you need to keep them from getting spongy anyway. Tree Fort had this on back order but shipped it out super quick as this is a tool for every home wrench.

As others have said, makes bleeding and setting up hydros a cinch, and a one person job.