Thumbpicks have a curl or loop to fit on the thumb, the curl then straightens out and projects down to form the plectrum part.
I went through a ton of thumb picks, cheap ones posh ones, some metal loops with hard plastic pick etc. In the end it came down to the cheap ones I started out with, with the pick part reshaped how I wanted. And heat & bend the loop so it fits (trial & error), one size definitely doesn't fit all.
After some gigs I got some skateboard grip tape, cut off a little piece and stuck it inside the loop. It's almost like sticky-back sandpaper, keeps the pick in place on a sweaty gig. I've still got a couple from about 9 years ago with the tape still on 'em. (Also amazing the pick monkeys haven't made the picks disappear like so many...)
edit - they're Planet Waves, a few squid for a 5-pack, but there's lots very similar poss the same but rebranded