I have not seen or used one of those but the word proprietary usually just means they can charge more for it. If what we are talking about here is a WINEGARD RPGM52 you can buy this exact item from AMAZON for around $28...
WINEGARD RPGM52 AMAZONgoogle image
If it looks like the AMAZON part shown above I would get one of those and make the connections and slip some one inch heat shrink material over both connections and heat shrink both ends. That will give you alot of strength against being pulled apart... You would want to slip the heat shrink tubing on the cable end before you connect both ends together. The heat shrink may not fit over the 12VDC plug on the other end to be slid all the way down to the antenna side where those two connectors are located. Then after I heat shrinked both end i would use some small tie wraps and tightned down on both ends of the heat shrink. This isn't going to be ripped off very easy...
You probably always have the 50-foot extension cord connected so it would not make any difference to make it more permanent connection. Seems silly to have to keep purchasing a new one every six months or so...
Just some of my thoughts here...
Roy Ken