Ariens 21 lawn mower gas tank cap

My lawn mower cap cracked, the top separating from the threads part. I tried gluing it and that did not hold. So my next attempt was to heat weld the parts together, and put metal inserts. That was definitely better than the glue (the cap is still usable), but not perfect: the two pieces separated a bit at the crack. The metal keeps them together, but it was annoying me that the crack was there. Last week I finally had my last attempt (getting a generic replacement from home depot) which also fa