It's LEFT HAND thread. Sure that's not your issue?
JB weld it - might be a little messy for such a small application
Epoxy it - maybe too messy as well
Hot glue - drill out the ID a little bit, glue it, then slam it on
Heat up the stripped ID a little, push it on and let it cure back on the stem?
Junk yard/eBay/Craigslist parts bike/etc. for a replacement
If the knob is actually broke and not stripped, you could try a knob from a lamp. Wrong thread (possibly find left hand thread lamp knobs??) but could be used with the above options.