Do you not have a petcock? or is your petcock bad. If you're being super stingy with a f'd up petcock, i've had luck in the past rebuilding them as an open-circuit - essentially removing the meat of the selector rubber (which is usually the cracked and broken piece) and replacing it with a proper sized cheap o-ring from a hardware store or whatever. Then you have essentially a bung.
Alternately I've fit all sorts of dumb things in there. if it's male threads from the tank, hose clamp a big soft rubber hose on there and get a cheap ass plastic reducer barb to get it to 1/16". If it's a female, see if you can squish some hose up in there enough to stick without leaking.
Another option is to use a plastic fitting bungs, like SAE or NPT or whatever, get something close but doesnt need to be the right metric size. Then just crossthread the shit out of it. it'll probably seal up enough to work and it won't wreck the threads.
But whatever you do, don't do anything that'd damage the threads - in case you ever wise up and want to skip an extra-value meal and buy a proper petcock with that money instead.