You can also take a grease nipple and put a rubber hose end on the threads of it, to form a good fit, when you pump the grease in.
I have actually seen lower units with grease nipples on them, but that’s not eally a good idea. It’ll catch weeds and the water could make a mess of the spring/check ball assembly inside. Now, if you had a grease nipple that would thread in finger tight, that would work perfectly. Then unscrew it when you’re done and put the plug back in.