This might make you smile. My 1300 Metro engine has the thermostat housing with the black plastic screw in plug in the top. Yesterday I was refilling the system after sorting out an old hose leak through the top of the housing and when I came to put the plug back in it broke off just under the flange ! Oh no...where the heck am I going to get one of them from? So, like many of our type I started ferreting through all those boxes and drawers in the the garage and.. Lo and behold....look what fits! ......
It's perfect, you are bleeding a heating/cooling system afterall. In a similar vein, I found that a Cambels soup tin is exactly the right diameter to put in the crankshaft oil seal next to the flywheel