I bought this kit of replacement metal buckles, and they work beautifully: https://www.amazon.com/gp/product/B07BDH...

This kit comes with four buckles, two for the left Joy Con, and two for the right. If you happen to have two sets (or plan to get a second set) of Joy Cons, a whopping $9.00 will set you up right.

I did have a bit of trouble replacing the metal cover that holds the buckle in place, but that was my own stupidity as I was trying to do this repair with insufficient light. Once I shed some light on the subject and I could see what I was doing, everything went back together smooth as silk. I replaced the buckles on both of my Joy Cons, and they’re both rock solid now.