Meh. Extra, messy, unnecessary step.
Bench job done right: you need a set of master cylinder plugs. New master cylinders usually ship with cheap plastic ones that will suffice. You're just looking to drip paint-clouding glycol on your car, your workbench, your tools, etc.
While common "gospel" says you must bench bleed before mounting, I simply refuse to do it this way, and will never try it this way again.
There is a better way...
Basically, the MC is shipped virtually dry, and will create a bit of an "airlock" if you will, due to the compressibility of air relative to the hydraulic brake fluid. An insane amount of bleeding often won't get you anywhere.
So... (I should charge for this, but today it's free) ...
Get a 3-6 foot length of polyurethane clear "fishtank" tubing, with about a 3/16" ID. The ID should fit nice and tight over the bleeder nipple on the closest brake caliper or cylinder, for us LHD folks in 'Merica that's the left front wheel. You'll need a length of tubing that can reach the master cylinder reservoir.
Fender covers are always recommended when working around good paint.
Now is your opportunity to take the reservoir off (if the new MC didn't come complete) and clean it thouroughly, with brake cleaner inside and out.
Once reinstalled and filled with fresh brake fluid, the loose end of the tubing should be inserted into the reservoir, and secured with duct tape or a zip tie in such a way that it can't pop out. The end of the tubing should be well submerged in new brake in the MC reservoir.
Crack the nipple with the tubing. Some cardboard under this arrangement might be useful, as it will still drip.
Now pump the brake pedal slowly and repeatedly, with full strokes from foot-off to floor, until you observe zero bubbles traveling through the tubing. Close the nipple, remove the tubing, wipe up the inevitable drip or two.
You may want to replace the bulk of the fluid in the MC at this point, if this process pushed a lot of older fluid out of the piston that appeared dark or discolored.
You can now bleed the entire brake system normally. This process takes less time than a bench bleed, usually results in less of a mess, and gives a clear visual indication when performed properly.