Search for the little spiral silicone ones! https://www.amazon.com/dp/B0757JLTY7?ref=ppx_pop_mob_ap_share Like those. They work well on thin and thicker bands. And it brings a bunch so you can cut them up and keep random spares with you in your purse, office, car...
Limited-time deal: Ring Size Adjuster for Loose Rings - 12 Pack, 2 Sizes - Jewelry Sizer, Mandrel for Making Jewelry Guard, Spacer, Sizer, Fitter - Spiral Silicone Tightener Set with Polishing Cloth https://www.amazon.com/dp/B0757JLTY7/ref=cm_sw_r_cp_api_glt_i_dl_FFE4M5VF0D4E8D9PE0A9?_encoding=UTF8&psc=1 they have these silicone loops to make your ring fit
Ring Size Adjuster for Loose Rings - 12 Pack, 2 Sizes - Jewelry Sizer, Mandrel for Making Jewelry Guard, Spacer, Sizer, Fitter - Spiral Silicone Tightener Set with Polishing Cloth https://www.amazon.com/dp/B0757JLTY7/ref=cm_sw_r_cp_api_i_dWlnFbRTVYA9N
I’d recommend the coils over the plastic snap on ones. I find they fit better and aren’t as obtrusive.
https://www.amazon.com/dp/B0757JLTY7/ref=cm_sw_r_cp_api_i_nQzeFbYDF7CGM