Radiator question

The radiator sprang a leak in the plastic around the upper hose outlet on my car, so I got on eBay and bought a new radiator. It got here, and I noticed it had an extra "hole" on the front side, and found out that some cars have a coolant level sensor gauge there. Alas, mine does not and I had a devil of a time getting the radiator to fit. But I wonder, the extra "hole" (for want of a better word) came with a plug in it, and I zip-tied the side grasps to (hopefully) keep the pressure from popping the plug out. But I would sure like to have a better way to seal up that "extra" hole. If I bought a coolant level sensor gauge to go in there and inserted it, and just did not even worry about hooking it up. Would it hold, and not start leaking? I want to get another 15,000 miles or so out of this old car, but I sure do not want to fool with the radiator anymore. Any advice, suggestions you can offer are greatly appreciated.
Warm regards.