Reusing vape batteries in flash lights? Is this a crazy idea.

So people throw out disposable vapes all the time. I cracked one open to take a look and it turns out they have a 18500 battery in it. So I was thinking of 3D printing a spacer to make it the same size as a 18650 and putting some shrink wrap on them. I just wanted to see what people who know more than me think. Is this a bad idea that is going to burn my house down? a waste of time perhaps?

I did a trial run in my wurkkos fc11 and they seem to work, it charges and looks just as bright.