I learned this trick years ago. If you have trouble with a metal thimble not fitting your finger, buy a cheap one and using a hammer hit it until the opening is oval shaped. It makes 100% difference to being comfortable fit.
I tried this and can't believe how much better the thimble feels and fits on my finger! I had a cheap metal thimble so I thought why not? I'm so glad I did. The oval shape is the shape of my finger. It didn't take but a couple of whacks to get it just right. Thanks!