Wheel Bearing Grease Packing Tool adapter / No Zert

Had a desire to repack wheel bearings with grease quicker than by disassembling the hub.
Of course, there are commercially available​ devices to accomplish the task of easily adding grease between the two bearings of the hub. All that I saw were manufactured from metal and had a grease fitting / zert installed. Some also included a back plate. All were more expensive than I was willing to pay for this type of tool and it's function.

I concluded the pressures could be contained with a 3D prin