mystery missing plug out of radiator in vovlo 240 1992
 

first time poster, please re-dreict me if this is the wrong place to post. 92 240 volvo, 5 speed, purchased from a 240 mechanic who's been awesome about helping me fix the various problems that crop up. driving yesterday i realized i was losing all my coolant and upon inspection, coolant was leaking out of a hole in the radiator, right below the hose coming in from the reservoir. theres a rubber seal around the hole, but no plug was to be found. being far from a parts store, i used an old spark plug, wrapped it in duct tape and shoved it in the hole, which is keeping most of the coolant in, but has a small leak still. does anyone know what this missing plug is? parts number or name? so far a cork has been advised as a good fix-it, as well as a salvage yard to find a replacement, but Im on my way to pick up a manual to see if i can get a little more technical with it. including picture- where the spark plug is poking out is the hole. not sure if radiator is original or a replacement. any help appreciated!